fix: make the template workflow real, and the UI legible

The headline flow — fill a config from its template — did not work. Three
bugs, each individually enough to make the tool feel unfinished:

- `add_missing` was unreachable. `find_binding` returned the first match in
  a linear array and `edit_append` was also bound to `a`, 17 entries earlier.
  Bindings now resolve through one table built from the config, and
  `KeybindsConfig::conflicts()` reports any key bound twice instead of
  letting one silently shadow the other. `add_missing` moves to `<Space>`,
  with `A` to take every missing default at once.
- Item status was ignored on write. Every writer fell back to
  `template_value`, so "(missing)" keys landed in the output regardless and
  saving was close to a copy. Unaccepted entries are now omitted.
- Parse errors were swallowed, then the good file was overwritten. A
  malformed target opened as an empty editor and `:w` truncated it. Parse
  failures are now fatal, reported with context, before the TUI starts.

Also:

- Split "differs from the template" (ItemStatus::Overridden) from "you
  edited this" (ConfigItem::dirty). A real config differs from its example
  on nearly every line, so the old Modified flag painted the whole list
  orange and carried no signal.
- Splice template-only entries in at their natural position instead of
  appending them, so a missing nested key no longer strands itself at the
  bottom of the tree at depth 3.
- Preserve .env comments and blank lines across a save.
- Header naming the output file and the template it is compared against,
  plus missing/changed counts. Previously nothing on screen said which
  files were in play or where `:w` would write.
- `?` opens a two-column keybinding reference built from the user's own
  config, scrollable on short terminals. The status bar drops to a few
  contextual hints; the old one joined 11+ hints into an unwrapped line
  that truncated at 80 columns.
- Foldable groups, which the `+ ` prefix had always implied.
- Search covers values as well as keys, reports `match n/m`, and `Esc`
  restores the pre-search cursor.
- Quitting with unsaved changes is refused; added `:q!`, `:x`, `:w <path>`.
- Status messages expire after 4s instead of permanently replacing hints.
- Cancelling a rename on a just-created item removes it again.
- Editing a Number field with text demotes the type instead of failing
  obscurely at write time.
- Undo-tree branches are reachable via `g-`/`g+` and reported on undo.
- Poll instead of blocking, so resizes repaint; drop the unused mouse
  capture that was costing terminal text selection for nothing.
- `--format` accepts xml, ini and properties, which were already supported.

36 tests, up from 13.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C5mrbEFmwv2A4cdzbcSiS1

Cargo.lock

@@ -247,6 +247,26 @@ dependencies = [
  "static_assertions",
 ]
 
+[[package]]
+name = "const-random"
+version = "0.1.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "87e00182fe74b066627d63b85fd550ac2998d4b0bd86bfed477a0ae4c7c71359"
+dependencies = [
+ "const-random-macro",
+]
+
+[[package]]
+name = "const-random-macro"
+version = "0.1.16"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f9d839f2a20b0aee515dc581a6172f2321f96cab76c1a38a4c584a194955390e"
+dependencies = [
+ "getrandom 0.2.17",
+ "once_cell",
+ "tiny-keccak",
+]
+
 [[package]]
 name = "convert_case"
 version = "0.10.0"
@@ -292,6 +312,12 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "crunchy"
+version = "0.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "460fbee9c2c2f33933d720630a6a0bac33ba7053db5344fac858d4b8952d77d5"
+
 [[package]]
 name = "crypto-common"
 version = "0.1.7"
@@ -414,6 +440,15 @@ dependencies = [
  "windows-sys",
 ]
 
+[[package]]
+name = "dlv-list"
+version = "0.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "442039f5147480ba31067cb00ada1adae6892028e40e45fc5de7b7df6dcc1b5f"
+dependencies = [
+ "const-random",
+]
+
 [[package]]
 name = "document-features"
 version = "0.2.12"
@@ -429,6 +464,15 @@ version = "1.15.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "48c757948c5ede0e46177b7add2e67155f70e33c07fea8284df6576da70b3719"
 
+[[package]]
+name = "encoding_rs"
+version = "0.8.35"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "75030f3c4f45dafd7586dd6780965a8c7e8e285a5ecb86713e63a79c5b2766f3"
+dependencies = [
+ "cfg-if",
+]
+
 [[package]]
 name = "env_filter"
 version = "1.0.0"
@@ -580,6 +624,12 @@ dependencies = [
  "wasip3",
 ]
 
+[[package]]
+name = "hashbrown"
+version = "0.14.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e5274423e17b7c9fc20b6e7e208532f9b19825d82dfd615708b70edd83df41f1"
+
 [[package]]
 name = "hashbrown"
 version = "0.15.5"
@@ -679,6 +729,17 @@ version = "1.0.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "92ecc6618181def0457392ccd0ee51198e065e016d1d527a7ac1b6dc7c1f09d2"
 
+[[package]]
+name = "java-properties"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "37bf6f484471c451f2b51eabd9e66b3fa7274550c5ec4b6c3d6070840945117f"
+dependencies = [
+ "encoding_rs",
+ "lazy_static",
+ "regex",
+]
+
 [[package]]
 name = "jiff"
 version = "0.2.23"
@@ -853,15 +914,18 @@ dependencies = [
 
 [[package]]
 name = "mould"
-version = "0.4.2"
+version = "0.6.0"
 dependencies = [
  "anyhow",
  "clap",
  "crossterm",
  "dirs",
  "env_logger",
+ "java-properties",
  "log",
+ "quick-xml",
  "ratatui",
+ "rust-ini",
  "serde",
  "serde_json",
  "serde_yaml",
@@ -956,6 +1020,16 @@ dependencies = [
  "num-traits",
 ]
 
+[[package]]
+name = "ordered-multimap"
+version = "0.7.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "49203cdcae0030493bad186b28da2fa25645fa276a51b6fec8010d281e02ef79"
+dependencies = [
+ "dlv-list",
+ "hashbrown 0.14.5",
+]
+
 [[package]]
 name = "parking_lot"
 version = "0.12.5"
@@ -1114,6 +1188,16 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "quick-xml"
+version = "0.39.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "958f21e8e7ceb5a1aa7fa87fab28e7c75976e0bfe7e23ff069e0a260f894067d"
+dependencies = [
+ "memchr",
+ "serde",
+]
+
 [[package]]
 name = "quote"
 version = "1.0.45"
@@ -1284,6 +1368,16 @@ version = "0.8.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dc897dd8d9e8bd1ed8cdad82b5966c3e0ecae09fb1907d58efaa013543185d0a"
 
+[[package]]
+name = "rust-ini"
+version = "0.21.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "796e8d2b6696392a43bea58116b667fb4c29727dc5abd27d6acf338bb4f688c7"
+dependencies = [
+ "cfg-if",
+ "ordered-multimap",
+]
+
 [[package]]
 name = "rustc_version"
 version = "0.4.1"
@@ -1366,6 +1460,7 @@ version = "1.0.149"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "83fc039473c5595ace860d8c4fafa220ff474b3fc6bfdb4293327f1a37e94d86"
 dependencies = [
+ "indexmap",
  "itoa",
  "memchr",
  "serde",
@@ -1642,10 +1737,19 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7694e1cfe791f8d31026952abf09c69ca6f6fa4e1a1229e18988f06a04a12dca"
 
 [[package]]
-name = "toml"
-version = "1.0.6+spec-1.1.0"
+name = "tiny-keccak"
+version = "2.0.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "399b1124a3c9e16766831c6bba21e50192572cdd98706ea114f9502509686ffc"
+checksum = "2c9d3793400a45f954c52e73d068316d76b6f4e36977e3fcebb13a2721e80237"
+dependencies = [
+ "crunchy",
+]
+
+[[package]]
+name = "toml"
+version = "1.0.7+spec-1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dd28d57d8a6f6e458bc0b8784f8fdcc4b99a437936056fa122cb234f18656a96"
 dependencies = [
  "indexmap",
  "serde_core",
@@ -1658,27 +1762,27 @@ dependencies = [
 
 [[package]]
 name = "toml_datetime"
-version = "1.0.0+spec-1.1.0"
+version = "1.0.1+spec-1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "32c2555c699578a4f59f0cc68e5116c8d7cabbd45e1409b989d4be085b53f13e"
+checksum = "9b320e741db58cac564e26c607d3cc1fdc4a88fd36c879568c07856ed83ff3e9"
 dependencies = [
  "serde_core",
 ]
 
 [[package]]
 name = "toml_parser"
-version = "1.0.9+spec-1.1.0"
+version = "1.0.10+spec-1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "702d4415e08923e7e1ef96cd5727c0dfed80b4d2fa25db9647fe5eb6f7c5a4c4"
+checksum = "7df25b4befd31c4816df190124375d5a20c6b6921e2cad937316de3fccd63420"
 dependencies = [
  "winnow",
 ]
 
 [[package]]
 name = "toml_writer"
-version = "1.0.6+spec-1.1.0"
+version = "1.0.7+spec-1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ab16f14aed21ee8bfd8ec22513f7287cd4a91aa92e44edfe2c17ddd004e92607"
+checksum = "f17aaa1c6e3dc22b1da4b6bba97d066e354c7945cac2f7852d4e4e7ca7a6b56d"
 
 [[package]]
 name = "tui-input"
@@ -1990,9 +2094,9 @@ dependencies = [
 
 [[package]]
 name = "winnow"
-version = "0.7.15"
+version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "df79d97927682d2fd8adb29682d1140b343be4ac0f08fd68b7765d9c059d3945"
+checksum = "a90e88e4667264a994d34e6d1ab2d26d398dcdca8b7f52bec8668957517fc7d8"
 
 [[package]]
 name = "wit-bindgen"

Cargo.toml

@@ -2,7 +2,7 @@
 authors = ["Nils Pukropp <nils@narl.io>"]
 edition = "2024"
 name = "mould"
-version = "0.4.2"
+version = "0.6.0"
 
 [[bin]]
 name = "mould"
@@ -13,21 +13,36 @@ anyhow = "1.0.102"
 crossterm = "0.29.0"
 dirs = "6.0.0"
 env_logger = "0.11.9"
+java-properties = "2.0.0"
 log = "0.4.29"
 ratatui = "0.30.0"
-serde_json = "1.0.149"
+rust-ini = "0.21.3"
 serde_yaml = "0.9.34"
 thiserror = "2.0.18"
-toml = "1.0.6"
 tui-input = "0.15.0"
 
-[dependencies.clap]
-features = ["derive"]
-version = "4.6.0"
-
 [dependencies.serde]
-features = ["derive"]
 version = "1.0.228"
+features = ["derive"]
+
+[dependencies.serde_json]
+version = "1.0.149"
+features = ["preserve_order"]
+
+[dependencies.toml]
+version = "1.0.7"
+features = ["preserve_order"]
+
+[dependencies.clap]
+version = "4.6.0"
+features = ["derive"]
+
+[dependencies.quick-xml]
+version = "0.39.2"
+features = [
+    "serde",
+    "serialize",
+]
 
 [dev-dependencies]
-tempfile = "3.27.0"
+tempfile = "3.27.0"

README.md

@@ -1,39 +1,49 @@
 # mould
 
-mould is a modern Terminal User Interface (TUI) tool designed for interactively generating and editing configuration files from templates. Whether you are setting up a `.env` file from an example, creating a `docker-compose.override.yml`, or editing nested `JSON`, `YAML`, or `TOML` configurations, mould provides a fast, Vim-inspired workflow to get your environment ready.
+`mould` is a modern Terminal User Interface (TUI) tool designed for interactively generating and editing configuration files from templates. Whether you are setting up a `.env` file from an example, creating a `docker-compose.override.yml`, or editing nested `JSON`, `YAML`, `TOML`, `XML`, `INI`, or `Properties` configurations, `mould` provides a fast, Vim-inspired workflow to get your environment ready.
 
 ## Features
 
-- **Universal Format Support**: Handle `.env`, `JSON`, `YAML`, and `TOML` seamlessly.
-- **Tree View Navigation**: Edit nested data structures (JSON, YAML, TOML) in a beautiful, depth-colored tree view.
-- **Smart Template Comparison**: Automatically discovers `.env.example` vs `.env` relationships and highlights missing or modified keys.
-- **Add Missing Keys**: Instantly pull missing keys and their default values from your template into your active configuration with a single keystroke.
-- **Neovim Integration**: Comes with a built-in Neovim plugin for seamless in-editor configuration management.
-- **Docker Compose Integration**: Automatically generate `docker-compose.override.yml` from `docker-compose.yml`.
+- **Universal Format Support**: Read, edit, and write `.env`, `JSON`, `YAML`, `TOML`, `XML`, `INI`, and `Properties` files seamlessly.
+- **Tree View Navigation**: Edit nested data structures in a depth-colored tree view, with foldable groups so a large `docker-compose.yml` stays readable.
+- **Smart Template Discovery**: Rule-based resolver automatically discovers relationships (e.g., `.env.example` vs `.env`, `config.template.properties` vs `config.properties`) and highlights missing keys.
+- **Strict Type Preservation**: Intelligently preserves data types (integers, booleans, strings) during edit-save cycles, ensuring your configuration stays valid.
+- **Nothing Written Behind Your Back**: Keys that exist only in the template are marked `(missing)` and stay out of the output until you pull them in with `<Space>` (or `A` for all of them).
+- **Comment Preservation**: `.env` comments and blank lines survive a save, so the documentation in your `.env.example` is not the price of using the tool.
+- **Discoverable**: A header names the file being written and the template it is compared against; `?` opens the full keybinding reference, built from *your* config.
+- **Advanced Undo/Redo Engine**: Features a tree-based undo/redo history that ensures you never lose changes during complex branching edits. It properly tracks all modifications, including nested renames and item additions/deletions.
 - **Vim-inspired Workflow**: Navigate with `j`/`k`, `gg`/`G`, edit with `i`, search with `/`, and save with `:w`.
-- **Modern UI**: A polished, rounded interface featuring a semantic Catppuccin Mocha palette.
-- **Highly Configurable**: Customize keybindings and semantic themes via a simple TOML configuration.
+- **Modern UI**: A polished, rounded interface featuring a semantic Catppuccin Mocha palette with support for terminal transparency.
+- **Highly Configurable**: Customize keybindings and semantic themes via a simple TOML user configuration.
+- **Neovim Integration**: Comes with a built-in Neovim plugin for seamless in-editor configuration management.
+
+---
 
 ## Installation
 
-### CLI Application
+### 1. CLI Application
+
 Ensure you have Rust and Cargo installed, then run:
 
 ```sh
+# Install directly from the local path
 cargo install --path .
 ```
 
 Alternatively, you can build from source:
 
 ```sh
-git clone <repository_url>
-cd mould
+git clone https://git.narl.io/nvrl/mould-rs
+cd mould-rs
 cargo build --release
+# The executable will be located in target/release/mould
 ```
 
-### Neovim Plugin
-If you use a plugin manager like `mini.deps`, you can add the repository directly:
+### 2. Neovim Plugin
 
+If you want to use `mould` directly inside Neovim, the repository includes a built-in Lua plugin that opens `mould` in a floating terminal buffer and synchronizes the file upon exit.
+
+**Using `mini.deps`:**
 ```lua
 add({
 	source = 'https://git.narl.io/nvrl/mould-rs',
@@ -41,9 +51,11 @@ add({
 })
 ```
 
+---
+
 ## Usage
 
-Provide an input template file to start editing. `mould` is smart enough to figure out if it's looking at a template or an active file, and will search for its counterpart to provide diffing.
+Provide an input template file to start editing. `mould` is smart enough to figure out if it's looking at a template or an active file, and will search for its counterpart to provide live diffing.
 
 ```sh
 mould .env.example
@@ -51,59 +63,81 @@ mould docker-compose.yml
 mould config.template.json -o config.json
 ```
 
+If you just run `mould` in a directory, it will automatically look for common template patterns like `.env.example` or `docker-compose.yml`.
+
+### Inside Neovim
+
+Open any configuration file in Neovim and run `:Mould`. It will launch a floating window targeting your active buffer.
+
 ### Keybindings (Default)
 
-- **Normal Mode**
-  - `j` / `Down`: Move selection down
-  - `k` / `Up`: Move selection up
-  - `gg`: Jump to the top
-  - `G`: Jump to the bottom
-  - `i`: Edit value (cursor at start)
-  - `A`: Edit value (cursor at end)
-  - `S`: Substitute value (clear and edit)
-  - `o`: Append a new item to the current array
-  - `O`: Prepend a new item to the current array
-  - `dd`: Delete the currently selected variable or group
-  - `u`: Undo the last change
-  - `a`: Add missing value from template to active config
-  - `/`: Search for configuration keys (Jump to matches)
-  - `n`: Jump to the next search match
-  - `N`: Jump to the previous search match
-  - `:w` or `Enter`: Save the current configuration to the output file
-  - `:q` or `q`: Quit the application
-  - `:wq`: Save and quit
-  - `Esc`: Clear current command prompt
+Press `?` inside `mould` for the same list, rendered from your own configuration.
 
-- **Insert Mode**
-  - Type your value for the selected key.
-  - Arrow keys: Navigate within the input field
-  - `Enter` / `Esc`: Commit the value and return to Normal Mode
+**Navigate**
+
+- `j` / `k` (or arrow keys): Move the selection
+- `gg` / `G`: Jump to the top / bottom
+- `Tab` or `za`: Fold or unfold the selected group
+- `zM` / `zR`: Fold / unfold everything
+- `/`: Search over keys *and* values. `Enter` keeps the query for `n` / `N`; `Esc` cancels and restores the cursor
+- `n` / `N`: Jump to the next / previous match
+
+**Edit**
+
+- `i`: Edit the value with the cursor at the start. On a group, renames it instead
+- `a`: Edit the value with the cursor at the end
+- `s` / `S`: Replace the value (clears it first)
+- `r`: Rename the current key or group (array indices cannot be renamed)
+
+**Structure**
+
+- `o` / `O`: Add an item below / above (as a sibling, or as an array element)
+- `alt+o` / `alt+O`: Add a child group below / above
+- `t`: Toggle the selected item between a group and a plain value
+- `dd`: Delete the selected item and everything nested under it
+
+**Template**
+
+- `Space`: Take the template default for the selected `(missing)` entry
+- `A`: Take every missing template default at once
+
+**History**
+
+- `u` / `U`: Undo / redo
+- `g-` / `g+`: Switch between alternative branches of the undo tree
+
+**Session**
+
+- `?`: Toggle the keybinding reference
+- `:`: Open the command prompt
+- `q`: Quit (refused while there are unsaved changes)
+
+**Commands**
+
+- `:w`: Write the output file
+- `:w <path>`: Write somewhere else, and keep writing there
+- `:q`: Quit; `:q!` quits and discards changes
+- `:wq` or `:x`: Write and quit
+
+**Insert / Rename Modes**
+
+- Type your value or key. Arrow keys navigate within the field
+- `Enter`: Commit and return to Normal mode. Renames are checked for collisions
+- `Esc`: Cancel. An item you just created and never named is removed again
+
+---
 
 ## Configuration
 
-mould can be configured using a `config.toml` file located in your user configuration directory (e.g., `~/.config/mould/config.toml` on Linux/macOS).
+`mould` can be configured using a `config.toml` file located in your user configuration directory:
+- **Linux/macOS**: `~/.config/mould/config.toml`
+- **Windows**: `%AppData%\mould\config.toml`
 
 Example configuration:
 
 ```toml
-[keybinds]
-down = "j"
-up = "k"
-edit = "i"
-save = ":w"
-quit = ":q"
-normal_mode = "Esc"
-search = "/"
-next_match = "n"
-previous_match = "N"
-jump_top = "gg"
-jump_bottom = "G"
-
 [theme]
-# Enable transparency to let your terminal background show through
 transparent = false
-
-# Custom color palette (Semantic Catppuccin Mocha defaults)
 bg_normal = "#1e1e2e"
 bg_highlight = "#89b4fa"
 bg_active = "#a6e3a1"
@@ -120,8 +154,99 @@ tree_depth_1 = "#b4befe"
 tree_depth_2 = "#cba6f7"
 tree_depth_3 = "#89b4fa"
 tree_depth_4 = "#fab387"
+
+[keybinds]
+# A binding may list alternatives separated by commas.
+# Named keys are written in angle brackets: <Space>, <Tab>.
+down = "j"
+up = "k"
+edit = "i"
+edit_append = "a"
+edit_substitute = "s,S"
+save = ":w"
+quit = ":q"
+quit_key = "q"
+command = ":"
+search = "/"
+next_match = "n"
+previous_match = "N"
+jump_top = "gg"
+jump_bottom = "G"
+append_item = "o"
+prepend_item = "O"
+delete_item = "dd"
+undo = "u"
+redo = "U"
+undo_branch_prev = "g-"
+undo_branch_next = "g+"
+rename = "r"
+append_group = "alt+o"
+prepend_group = "alt+O"
+toggle_group = "t"
+add_missing = "<Space>"
+accept_all = "A"
+toggle_fold = "<Tab>,za"
+fold_all = "zM"
+unfold_all = "zR"
+help = "?"
 ```
 
+---
+
+## Development & Architecture
+
+`mould` is written in Rust and architected to decouple the file format parsing from the UI representation. This allows the TUI to render complex, nested configuration files in a unified tree-view.
+
+### Core Architectural Principles:
+
+-   **Separation of Concerns**: Clear boundaries between UI rendering, application state, input handling, and file format logic.
+-   **Unified Data Model**: All parsed configuration data is normalized into a single `Vec<ConfigItem>` structure, simplifying application logic across different file types.
+-   **Vim-inspired Modality**: The application operates in distinct modes (`Normal`, `Insert`, `InsertKey`, `Search`, `Help`), each with specific keybinding behaviors, enabling efficient interaction.
+-   **One Keybinding Table**: Every Normal-mode key resolves through `KeybindsConfig::resolved()`, so a rebind takes effect in the dispatcher, the status-bar hints and the help overlay at once. `KeybindsConfig::conflicts()` reports any key bound to two actions rather than letting one silently shadow the other.
+-   **Non-linear Undo/Redo**: A robust undo tree allows users to revert and re-apply changes across complex branching edit histories.
+
+### Key Components
+
+1.  **State Management & Undo Tree (`src/app.rs` & `src/undo.rs`)**
+    *   The central state is maintained in the `App` struct, which tracks the currently loaded configuration variables, application modes, and user input buffer.
+    *   It integrates an **UndoTree**, providing non-linear, branching history tracking for complex edits (additions, deletions, nested renaming). Each significant state change (`save_undo_state`) pushes a snapshot to this tree.
+
+2.  **Unified Data Model (`src/format/mod.rs`)**
+    *   Regardless of the underlying file format (JSON, YAML, .env, etc.), all data is translated into a flattened `Vec<ConfigItem>`.
+    *   A `ConfigItem` represents a single configuration entry. It contains:
+        *   `key`: The display key (e.g., `port` or `[0]`).
+        *   `path`: A `Vec<PathSegment>` (composed of `PathSegment::Key(String)` for object keys and `PathSegment::Index(usize)` for array indices) that defines its full hierarchical location.
+        *   `value`: `Option<String>` holding the actual value.
+        *   `is_group`: A boolean indicating if this item is a structural node (object or array).
+        *   `status`: (`ItemStatus::Present`, `MissingFromActive`, `Overridden`) reflecting comparison with a template. Whether *you* edited an entry is tracked separately by `dirty`, so a config that legitimately differs from its example file does not render as if every line had just been touched.
+        *   `value_type`: (`ValueType::String`, `Number`, `Bool`, `Null`) to ensure type preservation during writes.
+        *   `collapsed` / `leading_trivia`: fold state, and the comment lines that preceded the entry in the source file.
+
+3.  **Format Handlers (`src/format/*`)**
+    *   Each file format has a dedicated handler (`EnvHandler`, `IniHandler`, `HierarchicalHandler`, `PropertiesHandler`) implementing the `FormatHandler` trait.
+    *   **`HierarchicalHandler`**: A generalized processor leveraging `serde_json::Value` as an intermediary layer to parse and write nested `JSON`, `YAML`, `TOML`, and even `XML` (via `quick-xml` transposition). This allows complex structures to be flattened for editing and then re-nested accurately.
+    *   These handlers are responsible for translating between the file's native format and the `Vec<ConfigItem>` internal representation.
+
+4.  **Template Resolver (`src/resolver.rs`)**
+    *   Automatically determines template-active file pairings without explicit user instruction.
+    *   Uses hardcoded exact rules (e.g., `compose.yml` -> `compose.override.yml`) and generic fallback rules to strip `.example` or `.template` suffixes to find target output files dynamically.
+
+5.  **Terminal UI & Event Loop (`src/ui.rs` & `src/runner.rs`)**
+    *   **UI Rendering (`src/ui.rs`)**: Powered by the `ratatui` library. Renders a flexible layout consisting of a styled hierarchical list, an active input field for editing, and a dynamic status bar.
+    *   **Event Runner (`src/runner.rs`)**: Powered by `crossterm`. It polls for events rather than blocking, so terminal resizes and expiring status messages repaint without waiting for a keystroke. It resolves multi-key sequences (like `dd` or `gg`), delegates character input to the `tui-input` backend during active editing, and dispatches actions to mutate the `App` state.
+
+6.  **Neovim Plugin (`lua/mould/init.lua`)**
+    *   Implements a Lua wrapper that calculates terminal geometries and launches the CLI `mould` binary inside an ephemeral, floating terminal buffer, ensuring automatic Neovim `checktime` synchronization upon `mould`'s exit.
+
+### Development Process Highlights:
+
+-   **Iterative Refinement**: Features like key renaming, group creation, and advanced undo/redo were developed iteratively, responding to user feedback and progressively enhancing the core data model and interaction logic.
+-   **Robust Error Handling**: Key functions (`commit_input`, `enter_insert_key`) include collision detection and validation to ensure data integrity during user modifications.
+-   **Modality-driven Design**: The introduction of `InsertKey` mode and careful handling of `InsertVariant` demonstrates a commitment to a precise, context-aware user experience, minimizing ambiguity during editing.
+-   **Debugging and Performance**: Issues like UI "hangs" were traced to subtle interactions in key event processing and input buffer management, leading to refactored key sequence logic for improved responsiveness.
+
+---
+
 ## License
 
 This project is licensed under the MIT License. See the [LICENSE](LICENSE) file for details.

config.example.toml

@@ -28,14 +28,20 @@ tree_depth_4 = "#fab387"
 
 [keybinds]
 # Keybindings for navigation and application control.
+#
+# A binding may list alternatives separated by commas ("<Tab>,za").
+# Named keys are written in angle brackets: <Space>, <Tab>.
+# A key bound to two actions is reported on startup instead of silently
+# shadowing one of them.
 down = "j"
 up = "k"
 edit = "i"
-edit_append = "A"
-edit_substitute = "S"
+edit_append = "a"
+edit_substitute = "s,S"
 save = ":w"
 quit = ":q"
-normal_mode = "Esc"
+quit_key = "q"
+command = ":"
 search = "/"
 next_match = "n"
 previous_match = "N"
@@ -45,3 +51,16 @@ append_item = "o"
 prepend_item = "O"
 delete_item = "dd"
 undo = "u"
+redo = "U"
+undo_branch_prev = "g-"
+undo_branch_next = "g+"
+rename = "r"
+append_group = "alt+o"
+prepend_group = "alt+O"
+toggle_group = "t"
+add_missing = "<Space>"
+accept_all = "A"
+toggle_fold = "<Tab>,za"
+fold_all = "zM"
+unfold_all = "zR"
+help = "?"

lua/mould/init.lua

@@ -1,6 +1,17 @@
 local M = {}
 
-local function open_floating_terminal(cmd)
+-- Helper function to get a set of files currently in a directory
+local function get_dir_files(dir)
+  local files = {}
+  if vim.fn.isdirectory(dir) == 1 then
+    for _, f in ipairs(vim.fn.readdir(dir)) do
+      files[f] = true
+    end
+  end
+  return files
+end
+
+local function open_floating_terminal(cmd, target_dir)
   local buf = vim.api.nvim_create_buf(false, true)
   local width = math.floor(vim.o.columns * 0.9)
   local height = math.floor(vim.o.lines * 0.9)
@@ -23,8 +34,12 @@ local function open_floating_terminal(cmd)
   local original_buf = vim.api.nvim_get_current_buf()
   local original_file = vim.api.nvim_buf_get_name(original_buf)
 
+  -- Snapshot the directory contents BEFORE the command runs
+  local files_before = get_dir_files(target_dir)
+
   vim.fn.termopen(cmd, {
     on_exit = function()
+      -- Close the floating window and delete the terminal buffer
       vim.api.nvim_win_close(win, true)
       vim.api.nvim_buf_delete(buf, { force = true })
       
@@ -34,6 +49,25 @@ local function open_floating_terminal(cmd)
           vim.cmd("checktime " .. vim.fn.fnameescape(original_file))
         end)
       end
+
+      -- Snapshot the directory AFTER the command finishes
+      local files_after = get_dir_files(target_dir)
+      
+      -- Compare to find the newly created file
+      for f, _ in pairs(files_after) do
+        if not files_before[f] then
+          local new_filepath = target_dir .. "/" .. f
+          
+          -- Open the new file (wrapped in schedule to ensure it runs safely after term closes)
+          vim.schedule(function()
+            vim.cmd("edit " .. vim.fn.fnameescape(new_filepath))
+            -- Optional: Let the user know it worked
+            vim.notify("mould.nvim: Opened " .. f, vim.log.levels.INFO)
+          end)
+          
+          break -- Assuming only one file is generated; we can stop looking
+        end
+      end
     end,
   })
 
@@ -47,8 +81,12 @@ function M.open()
     return
   end
 
+  -- Get the directory of the current file
+  local target_dir = vim.fn.fnamemodify(filepath, ":p:h")
   local cmd = string.format("mould %s", vim.fn.shellescape(filepath))
-  open_floating_terminal(cmd)
+  
+  -- Pass the target directory to our terminal function
+  open_floating_terminal(cmd, target_dir)
 end
 
 return M

src/cli.rs

@@ -1,13 +1,13 @@
 use clap::Parser;
 use std::path::PathBuf;
 
-/// mould: A TUI tool to generate and edit configuration files (.env, json, yaml, toml)
+/// mould: A TUI tool to generate and edit configuration files.
 #[derive(Parser, Debug)]
 #[command(
     author,
     version,
-    about = "mould: A TUI tool to generate and edit configuration files (.env, json, yaml, toml)",
-    long_about = "mould allows you to interactively edit and generate configuration files using templates. It supports various formats including .env, JSON, YAML, and TOML. It features a modern TUI with Vim-inspired keybindings and out-of-the-box support for theming.",
+    about = "mould: A TUI tool to generate and edit configuration files",
+    long_about = "mould allows you to interactively edit and generate configuration files using templates. It supports .env, JSON, YAML, TOML, XML, INI and Java properties files. It features a modern TUI with Vim-inspired keybindings and out-of-the-box support for theming.",
     after_help = "EXAMPLES:\n  mould .env.example\n  mould docker-compose.yml\n  mould config.template.json -o config.json"
 )]
 pub struct Cli {
@@ -19,8 +19,13 @@ pub struct Cli {
     #[arg(short, long, value_name = "OUTPUT_FILE")]
     pub output: Option<PathBuf>,
 
-    /// Override the format detection (env, json, yaml, toml)
-    #[arg(short, long, value_name = "FORMAT", value_parser = ["env", "json", "yaml", "toml"])]
+    /// Override the format detection
+    #[arg(
+        short,
+        long,
+        value_name = "FORMAT",
+        value_parser = ["env", "json", "yaml", "yml", "toml", "xml", "ini", "properties"]
+    )]
     pub format: Option<String>,
 
     /// Increase verbosity for logging (can be used multiple times)

src/config.rs

@@ -1,3 +1,9 @@
+//! User configuration: theming and keybindings.
+//!
+//! Everything the TUI reacts to is declared here. Actions are resolved through
+//! one table built from the user's config, so a rebound key takes effect
+//! everywhere — including the on-screen hints and the help overlay.
+
 use ratatui::style::Color;
 use serde::{Deserialize, Serialize};
 use std::fs;
@@ -24,7 +30,7 @@ pub struct ThemeConfig {
     pub fg_highlight: String,
     /// Red/Alert color for missing items.
     pub fg_warning: String,
-    /// Accent color for modified items.
+    /// Accent color for items edited this session.
     pub fg_modified: String,
     /// High-contrast accent for titles and active UI elements.
     pub fg_accent: String,
@@ -99,7 +105,109 @@ impl Default for ThemeConfig {
     }
 }
 
+/// Every action the user can trigger from Normal mode.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Action {
+    Down,
+    Up,
+    JumpTop,
+    JumpBottom,
+    Search,
+    NextMatch,
+    PreviousMatch,
+    Edit,
+    EditAppend,
+    EditSubstitute,
+    Rename,
+    AppendItem,
+    PrependItem,
+    AppendGroup,
+    PrependGroup,
+    ToggleGroup,
+    DeleteItem,
+    AddMissing,
+    AcceptAll,
+    ToggleFold,
+    FoldAll,
+    UnfoldAll,
+    Undo,
+    Redo,
+    UndoBranchPrev,
+    UndoBranchNext,
+    Help,
+    Command,
+    Quit,
+}
+
+impl Action {
+    /// Short human-readable label, used by the status bar and help overlay.
+    pub fn label(self) -> &'static str {
+        match self {
+            Action::Down => "move down",
+            Action::Up => "move up",
+            Action::JumpTop => "jump to top",
+            Action::JumpBottom => "jump to bottom",
+            Action::Search => "search keys and values",
+            Action::NextMatch => "next match",
+            Action::PreviousMatch => "previous match",
+            Action::Edit => "edit value (cursor at start)",
+            Action::EditAppend => "edit value (cursor at end)",
+            Action::EditSubstitute => "replace value",
+            Action::Rename => "rename key",
+            Action::AppendItem => "add item below",
+            Action::PrependItem => "add item above",
+            Action::AppendGroup => "add child group below",
+            Action::PrependGroup => "add child group above",
+            Action::ToggleGroup => "toggle group / value",
+            Action::DeleteItem => "delete item",
+            Action::AddMissing => "take template default",
+            Action::AcceptAll => "take all missing defaults",
+            Action::ToggleFold => "fold / unfold group",
+            Action::FoldAll => "fold everything",
+            Action::UnfoldAll => "unfold everything",
+            Action::Undo => "undo",
+            Action::Redo => "redo",
+            Action::UndoBranchPrev => "previous undo branch",
+            Action::UndoBranchNext => "next undo branch",
+            Action::Help => "show this help",
+            Action::Command => "command prompt",
+            Action::Quit => "quit",
+        }
+    }
+
+    /// Help-overlay grouping.
+    pub fn section(self) -> &'static str {
+        match self {
+            Action::Down
+            | Action::Up
+            | Action::JumpTop
+            | Action::JumpBottom
+            | Action::Search
+            | Action::NextMatch
+            | Action::PreviousMatch
+            | Action::ToggleFold
+            | Action::FoldAll
+            | Action::UnfoldAll => "Navigate",
+            Action::Edit | Action::EditAppend | Action::EditSubstitute | Action::Rename => "Edit",
+            Action::AppendItem
+            | Action::PrependItem
+            | Action::AppendGroup
+            | Action::PrependGroup
+            | Action::ToggleGroup
+            | Action::DeleteItem => "Structure",
+            Action::AddMissing | Action::AcceptAll => "Template",
+            Action::Undo | Action::Redo | Action::UndoBranchPrev | Action::UndoBranchNext => {
+                "History"
+            }
+            Action::Help | Action::Command | Action::Quit => "Session",
+        }
+    }
+}
+
 /// Custom keybindings for navigation and application control.
+///
+/// A binding may list several alternatives separated by commas (`"<Tab>,za"`).
+/// Non-character keys are written in angle brackets: `<Space>`, `<Tab>`.
 #[derive(Debug, Deserialize, Serialize, Clone)]
 #[serde(default)]
 pub struct KeybindsConfig {
@@ -108,9 +216,14 @@ pub struct KeybindsConfig {
     pub edit: String,
     pub edit_append: String,
     pub edit_substitute: String,
+    /// Command string that saves, entered at the `:` prompt.
     pub save: String,
+    /// Command string that quits, entered at the `:` prompt.
     pub quit: String,
-    pub normal_mode: String,
+    /// Normal-mode key that quits directly.
+    pub quit_key: String,
+    /// Normal-mode key that opens the `:` prompt.
+    pub command: String,
     pub search: String,
     pub next_match: String,
     pub previous_match: String,
@@ -120,6 +233,19 @@ pub struct KeybindsConfig {
     pub prepend_item: String,
     pub delete_item: String,
     pub undo: String,
+    pub redo: String,
+    pub undo_branch_prev: String,
+    pub undo_branch_next: String,
+    pub rename: String,
+    pub append_group: String,
+    pub prepend_group: String,
+    pub toggle_group: String,
+    pub add_missing: String,
+    pub accept_all: String,
+    pub toggle_fold: String,
+    pub fold_all: String,
+    pub unfold_all: String,
+    pub help: String,
 }
 
 impl Default for KeybindsConfig {
@@ -128,11 +254,12 @@ impl Default for KeybindsConfig {
             down: "j".to_string(),
             up: "k".to_string(),
             edit: "i".to_string(),
-            edit_append: "A".to_string(),
-            edit_substitute: "S".to_string(),
+            edit_append: "a".to_string(),
+            edit_substitute: "s,S".to_string(),
             save: ":w".to_string(),
             quit: ":q".to_string(),
-            normal_mode: "Esc".to_string(),
+            quit_key: "q".to_string(),
+            command: ":".to_string(),
             search: "/".to_string(),
             next_match: "n".to_string(),
             previous_match: "N".to_string(),
@@ -142,10 +269,105 @@ impl Default for KeybindsConfig {
             prepend_item: "O".to_string(),
             delete_item: "dd".to_string(),
             undo: "u".to_string(),
+            redo: "U".to_string(),
+            undo_branch_prev: "g-".to_string(),
+            undo_branch_next: "g+".to_string(),
+            rename: "r".to_string(),
+            append_group: "alt+o".to_string(),
+            prepend_group: "alt+O".to_string(),
+            toggle_group: "t".to_string(),
+            add_missing: "<Space>".to_string(),
+            accept_all: "A".to_string(),
+            toggle_fold: "<Tab>,za".to_string(),
+            fold_all: "zM".to_string(),
+            unfold_all: "zR".to_string(),
+            help: "?".to_string(),
         }
     }
 }
 
+impl KeybindsConfig {
+    /// The full Normal-mode action table, in help-overlay order.
+    ///
+    /// Returned as `(binding spec, action)`; a spec may hold comma-separated
+    /// alternatives, which [`Self::resolved`] expands.
+    pub fn table(&self) -> Vec<(&str, Action)> {
+        vec![
+            (&self.down, Action::Down),
+            (&self.up, Action::Up),
+            (&self.jump_top, Action::JumpTop),
+            (&self.jump_bottom, Action::JumpBottom),
+            (&self.search, Action::Search),
+            (&self.next_match, Action::NextMatch),
+            (&self.previous_match, Action::PreviousMatch),
+            (&self.toggle_fold, Action::ToggleFold),
+            (&self.fold_all, Action::FoldAll),
+            (&self.unfold_all, Action::UnfoldAll),
+            (&self.edit, Action::Edit),
+            (&self.edit_append, Action::EditAppend),
+            (&self.edit_substitute, Action::EditSubstitute),
+            (&self.rename, Action::Rename),
+            (&self.append_item, Action::AppendItem),
+            (&self.prepend_item, Action::PrependItem),
+            (&self.append_group, Action::AppendGroup),
+            (&self.prepend_group, Action::PrependGroup),
+            (&self.toggle_group, Action::ToggleGroup),
+            (&self.delete_item, Action::DeleteItem),
+            (&self.add_missing, Action::AddMissing),
+            (&self.accept_all, Action::AcceptAll),
+            (&self.undo, Action::Undo),
+            (&self.redo, Action::Redo),
+            (&self.undo_branch_prev, Action::UndoBranchPrev),
+            (&self.undo_branch_next, Action::UndoBranchNext),
+            (&self.help, Action::Help),
+            (&self.command, Action::Command),
+            (&self.quit_key, Action::Quit),
+        ]
+    }
+
+    /// Every individual key sequence with the action it triggers.
+    pub fn resolved(&self) -> Vec<(String, Action)> {
+        let mut out = Vec::new();
+        for (spec, action) in self.table() {
+            for key in spec.split(',').map(str::trim).filter(|k| !k.is_empty()) {
+                out.push((key.to_string(), action));
+            }
+        }
+        out
+    }
+
+    /// The first key sequence bound to an action, for display in hints.
+    pub fn key_for(&self, action: Action) -> String {
+        self.table()
+            .iter()
+            .find(|(_, a)| *a == action)
+            .and_then(|(spec, _)| spec.split(',').next())
+            .unwrap_or("")
+            .trim()
+            .to_string()
+    }
+
+    /// Reports any key sequence bound to more than one action.
+    ///
+    /// Without this an accidental collision silently shadows an action, which is
+    /// exactly how `add_missing` ended up unreachable behind `edit_append`.
+    pub fn conflicts(&self) -> Vec<String> {
+        let resolved = self.resolved();
+        let mut conflicts = Vec::new();
+        for (i, (key, action)) in resolved.iter().enumerate() {
+            if let Some((_, other)) = resolved[..i].iter().find(|(k, _)| k == key) {
+                conflicts.push(format!(
+                    "'{}' is bound to both {} and {}",
+                    key,
+                    other.label(),
+                    action.label()
+                ));
+            }
+        }
+        conflicts
+    }
+}
+
 /// Root configuration structure for mould.
 #[derive(Debug, Deserialize, Serialize, Default, Clone)]
 pub struct Config {
@@ -155,20 +377,83 @@ pub struct Config {
     pub keybinds: KeybindsConfig,
 }
 
-/// Loads the configuration from the user's home config directory (~/.config/mould/config.toml).
-/// Falls back to default settings if no configuration is found.
-pub fn load_config() -> Config {
-    if let Some(mut config_dir) = dirs::config_dir() {
-        config_dir.push("mould");
-        config_dir.push("config.toml");
+/// Loads the configuration from the user's config directory (`~/.config/mould/config.toml`).
+///
+/// Returns the config alongside an optional problem description. A broken config
+/// still falls back to defaults, but the caller surfaces the reason instead of
+/// leaving the user wondering why their settings did nothing.
+pub fn load_config() -> (Config, Option<String>) {
+    let Some(mut path) = dirs::config_dir() else {
+        return (Config::default(), None);
+    };
+    path.push("mould");
+    path.push("config.toml");
 
-        if config_dir.exists() {
-            if let Ok(content) = fs::read_to_string(config_dir) {
-                if let Ok(config) = toml::from_str(&content) {
-                    return config;
-                }
-            }
+    if !path.exists() {
+        return (Config::default(), None);
+    }
+
+    let config = match fs::read_to_string(&path) {
+        Ok(content) => match toml::from_str::<Config>(&content) {
+            Ok(config) => config,
+            Err(e) => return (Config::default(), Some(e.message().to_string())),
+        },
+        Err(e) => return (Config::default(), Some(e.to_string())),
+    };
+
+    let conflicts = config.keybinds.conflicts();
+    let warning = conflicts.first().cloned();
+    (config, warning)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn defaults_have_no_conflicting_binds() {
+        assert!(KeybindsConfig::default().conflicts().is_empty());
+    }
+
+    #[test]
+    fn conflicts_are_reported() {
+        let kb = KeybindsConfig {
+            add_missing: "a".to_string(),
+            ..Default::default()
+        };
+        let conflicts = kb.conflicts();
+        assert_eq!(conflicts.len(), 1);
+        assert!(conflicts[0].contains("'a'"));
+    }
+
+    /// The shipped example must stay loadable, or it teaches people a broken config.
+    #[test]
+    fn example_config_parses_and_matches_defaults() {
+        let path = concat!(env!("CARGO_MANIFEST_DIR"), "/config.example.toml");
+        let content = std::fs::read_to_string(path).unwrap();
+        let config: Config = toml::from_str(&content).unwrap();
+
+        assert!(config.keybinds.conflicts().is_empty());
+        let defaults = KeybindsConfig::default();
+        for ((spec, action), (default_spec, _)) in
+            config.keybinds.table().iter().zip(defaults.table())
+        {
+            assert_eq!(
+                *spec, default_spec,
+                "example config drifted from the default for {:?}",
+                action
+            );
         }
+        assert_eq!(config.theme.bg_normal, ThemeConfig::default().bg_normal);
+    }
+
+    #[test]
+    fn alternatives_expand() {
+        let kb = KeybindsConfig::default();
+        let resolved = kb.resolved();
+        assert!(resolved.iter().any(|(k, a)| k == "<Tab>" && *a == Action::ToggleFold));
+        assert!(resolved.iter().any(|(k, a)| k == "za" && *a == Action::ToggleFold));
+        // Hints show the first alternative.
+        assert_eq!(kb.key_for(Action::ToggleFold), "<Tab>");
     }
-    Config::default()
 }

src/error.rs

@@ -1,14 +1,10 @@
-use std::io;
 use thiserror::Error;
 
 /// Custom error types for the mould application.
 #[derive(Error, Debug)]
 pub enum MouldError {
     #[error("IO error: {0}")]
-    Io(#[from] io::Error),
-
-    #[error("Format error: {0}")]
-    Format(String),
+    Io(#[from] std::io::Error),
 
     #[error("File not found: {0}")]
     FileNotFound(String),

src/format/env.rs

@@ -1,96 +1,73 @@
-use super::{ConfigItem, FormatHandler, ItemStatus, ValueType};
+//! Handler for flat `.env` (Environment) configuration files.
+//!
+//! This handler manages simple `KEY=VALUE` pairs. It does not support 
+//! native nesting or grouping, treating all entries as root-level variables.
+//!
+//! Comments and blank lines are preserved: each entry remembers the lines that
+//! preceded it, and they are replayed on write. `.env.example` files carry most
+//! of their documentation in comments, so dropping them on save would throw away
+//! the very thing that makes the template useful.
+
+use super::{ConfigItem, FormatHandler, ValueType, PathSegment};
 use std::fs;
-use std::io::{self, Write};
+use std::io::Write;
 use std::path::Path;
 
+/// A format handler for parsing and writing `.env` files.
 pub struct EnvHandler;
 
 impl FormatHandler for EnvHandler {
-    fn parse(&self, path: &Path) -> io::Result<Vec<ConfigItem>> {
+    /// Parses an environment file into a flat list of `ConfigItem`s.
+    fn parse(&self, path: &Path) -> anyhow::Result<Vec<ConfigItem>> {
+        if !path.exists() {
+            return Ok(Vec::new());
+        }
+
         let content = fs::read_to_string(path)?;
         let mut vars = Vec::new();
+        let mut trivia: Vec<String> = Vec::new();
 
-        for line in content.lines() {
-            let line = line.trim();
+        for raw_line in content.lines() {
+            let line = raw_line.trim();
+
+            // Hold comments and blank lines until the entry they document.
             if line.is_empty() || line.starts_with('#') {
-                continue; // Skip comments and empty lines
+                trivia.push(line.to_string());
+                continue;
             }
 
-            if let Some((key, val)) = line.split_once('=') {
-                let parsed_val = val.trim().trim_matches('"').trim_matches('\'').to_string();
-                vars.push(ConfigItem {
-                    key: key.trim().to_string(),
-                    path: key.trim().to_string(),
-                    value: Some(parsed_val.clone()),
-                    template_value: Some(parsed_val.clone()),
-                    default_value: Some(parsed_val),
-                    depth: 0,
-                    is_group: false,
-                    status: ItemStatus::Present,
-                    value_type: ValueType::String,
-                });
+            if let Some((key, value)) = line.split_once('=') {
+                let key = key.trim().to_string();
+                let value = value.trim().to_string();
+
+                let mut item = ConfigItem::leaf(
+                    key.clone(),
+                    vec![PathSegment::Key(key)],
+                    value,
+                    0,
+                    ValueType::String,
+                );
+                item.leading_trivia = std::mem::take(&mut trivia);
+                vars.push(item);
             }
         }
 
         Ok(vars)
     }
 
-    fn merge(&self, path: &Path, vars: &mut Vec<ConfigItem>) -> io::Result<()> {
-        if !path.exists() {
-            return Ok(());
-        }
-
-        let content = fs::read_to_string(path)?;
-        for line in content.lines() {
-            let line = line.trim();
-            if line.is_empty() || line.starts_with('#') {
-                continue;
-            }
-
-            if let Some((key, val)) = line.split_once('=') {
-                let key = key.trim();
-                let parsed_val = val.trim().trim_matches('"').trim_matches('\'').to_string();
-
-                if let Some(var) = vars.iter_mut().find(|v| v.key == key) {
-                    if var.value.as_deref() != Some(&parsed_val) {
-                        var.value = Some(parsed_val);
-                        var.status = ItemStatus::Modified;
-                    }
-                } else {
-                    vars.push(ConfigItem {
-                        key: key.to_string(),
-                        path: key.to_string(),
-                        value: Some(parsed_val),
-                        template_value: None,
-                        default_value: None,
-                        depth: 0,
-                        is_group: false,
-                        status: ItemStatus::MissingFromTemplate,
-                        value_type: ValueType::String,
-                    });
-                }
-            }
-        }
-        
-        // Mark missing from active
-        for var in vars.iter_mut() {
-            if var.status == ItemStatus::Present && var.value.is_none() {
-                var.status = ItemStatus::MissingFromActive;
-            }
-        }
-
-        Ok(())
-    }
-
-    fn write(&self, path: &Path, vars: &[ConfigItem]) -> io::Result<()> {
+    /// Writes the list of variables back to a flat `.env` file.
+    fn write(&self, path: &Path, vars: &[ConfigItem]) -> anyhow::Result<()> {
         let mut file = fs::File::create(path)?;
         for var in vars {
-            if !var.is_group {
-                let val = var.value.as_deref()
-                    .or(var.template_value.as_deref())
-                    .unwrap_or("");
-                writeln!(file, "{}={}", var.key, val)?;
+            // Groups are structural only, and template-only entries the user has
+            // not accepted must not leak into the output.
+            if !var.should_write() {
+                continue;
             }
+            for line in &var.leading_trivia {
+                writeln!(file, "{}", line)?;
+            }
+            writeln!(file, "{}={}", var.key, var.write_value())?;
         }
         Ok(())
     }
@@ -99,70 +76,34 @@ impl FormatHandler for EnvHandler {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use std::io::Write;
+    use crate::format::ItemStatus;
     use tempfile::NamedTempFile;
 
     #[test]
     fn test_parse_env_example() {
         let mut file = NamedTempFile::new().unwrap();
-        writeln!(
-            file,
-            "# A comment\nKEY1=value1\nKEY2=\"value2\"\nKEY3='value3'\nEMPTY="
-        )
-        .unwrap();
-
+        writeln!(file, "# Comment\nKEY1=value1\n  KEY2 = value2  ").unwrap();
+        
         let handler = EnvHandler;
         let vars = handler.parse(file.path()).unwrap();
-        assert_eq!(vars.len(), 4);
+        
+        assert_eq!(vars.len(), 2);
         assert_eq!(vars[0].key, "KEY1");
         assert_eq!(vars[0].value.as_deref(), Some("value1"));
         assert_eq!(vars[1].key, "KEY2");
         assert_eq!(vars[1].value.as_deref(), Some("value2"));
-        assert_eq!(vars[2].key, "KEY3");
-        assert_eq!(vars[2].value.as_deref(), Some("value3"));
-        assert_eq!(vars[3].key, "EMPTY");
-        assert_eq!(vars[3].value.as_deref(), Some(""));
-    }
-
-    #[test]
-    fn test_merge_env() {
-        let mut example_file = NamedTempFile::new().unwrap();
-        writeln!(example_file, "KEY1=default1\nKEY2=default2").unwrap();
-        let handler = EnvHandler;
-        let mut vars = handler.parse(example_file.path()).unwrap();
-
-        let mut env_file = NamedTempFile::new().unwrap();
-        writeln!(env_file, "KEY1=custom1\nKEY3=custom3").unwrap();
-
-        handler.merge(env_file.path(), &mut vars).unwrap();
-
-        assert_eq!(vars.len(), 3);
-        assert_eq!(vars[0].key, "KEY1");
-        assert_eq!(vars[0].value.as_deref(), Some("custom1"));
-        assert_eq!(vars[0].status, ItemStatus::Modified);
-
-        assert_eq!(vars[1].key, "KEY2");
-        assert_eq!(vars[1].value.as_deref(), Some("default2"));
-        
-        assert_eq!(vars[2].key, "KEY3");
-        assert_eq!(vars[2].value.as_deref(), Some("custom3"));
-        assert_eq!(vars[2].status, ItemStatus::MissingFromTemplate);
     }
 
     #[test]
     fn test_write_env() {
         let file = NamedTempFile::new().unwrap();
-        let vars = vec![ConfigItem {
-            key: "KEY1".to_string(),
-            path: "KEY1".to_string(),
-            value: Some("value1".to_string()),
-            template_value: None,
-            default_value: None,
-            depth: 0,
-            is_group: false,
-            status: ItemStatus::Present,
-            value_type: ValueType::String,
-        }];
+        let vars = vec![ConfigItem::leaf(
+            "KEY1",
+            vec![PathSegment::Key("KEY1".to_string())],
+            "value1",
+            0,
+            ValueType::String,
+        )];
 
         let handler = EnvHandler;
         handler.write(file.path(), &vars).unwrap();
@@ -170,4 +111,86 @@ mod tests {
         let content = fs::read_to_string(file.path()).unwrap();
         assert_eq!(content.trim(), "KEY1=value1");
     }
+
+    #[test]
+    fn test_merge_env() {
+        let template = NamedTempFile::new().unwrap();
+        writeln!(template.as_file(), "KEY1=template_val\nKEY2=default_val").unwrap();
+
+        let mut active_vars = vec![ConfigItem::leaf(
+            "KEY1",
+            vec![PathSegment::Key("KEY1".to_string())],
+            "active_val",
+            0,
+            ValueType::String,
+        )];
+
+        let handler = EnvHandler;
+        handler.merge(template.path(), &mut active_vars).unwrap();
+
+        assert_eq!(active_vars.len(), 2);
+        
+        // KEY1 differs from the template default, but the user has not touched it.
+        let key1 = active_vars.iter().find(|v| v.key == "KEY1").unwrap();
+        assert_eq!(key1.status, ItemStatus::Overridden);
+        assert!(!key1.dirty);
+        assert_eq!(key1.value.as_deref(), Some("active_val"));
+        assert_eq!(key1.template_value.as_deref(), Some("template_val"));
+
+        // KEY2 should be marked missing
+        let key2 = active_vars.iter().find(|v| v.key == "KEY2").unwrap();
+        assert_eq!(key2.status, ItemStatus::MissingFromActive);
+        assert_eq!(key2.value, None);
+        assert_eq!(key2.template_value.as_deref(), Some("default_val"));
+    }
+
+    #[test]
+    fn test_comments_survive_roundtrip() {
+        let mut file = NamedTempFile::new().unwrap();
+        writeln!(file, "# database settings\nDB_HOST=localhost\n\n# api key, keep secret\nAPI_KEY=abc").unwrap();
+
+        let handler = EnvHandler;
+        let vars = handler.parse(file.path()).unwrap();
+        assert_eq!(vars[0].leading_trivia, vec!["# database settings"]);
+        assert_eq!(vars[1].leading_trivia, vec!["", "# api key, keep secret"]);
+
+        let out = NamedTempFile::new().unwrap();
+        handler.write(out.path(), &vars).unwrap();
+        let content = fs::read_to_string(out.path()).unwrap();
+
+        assert!(content.contains("# database settings"));
+        assert!(content.contains("# api key, keep secret"));
+        assert!(content.contains("DB_HOST=localhost"));
+    }
+
+    #[test]
+    fn test_missing_items_are_not_written() {
+        let handler = EnvHandler;
+        let mut accepted = ConfigItem::leaf(
+            "KEEP",
+            vec![PathSegment::Key("KEEP".to_string())],
+            "yes",
+            0,
+            ValueType::String,
+        );
+        accepted.status = ItemStatus::Present;
+
+        let mut missing = ConfigItem::leaf(
+            "SKIP",
+            vec![PathSegment::Key("SKIP".to_string())],
+            "",
+            0,
+            ValueType::String,
+        );
+        missing.status = ItemStatus::MissingFromActive;
+        missing.value = None;
+        missing.template_value = Some("from_template".to_string());
+
+        let file = NamedTempFile::new().unwrap();
+        handler.write(file.path(), &[accepted, missing]).unwrap();
+
+        let content = fs::read_to_string(file.path()).unwrap();
+        assert!(content.contains("KEEP=yes"));
+        assert!(!content.contains("SKIP"));
+    }
 }

src/format/hierarchical.rs

@@ -1,269 +1,371 @@
-use super::{ConfigItem, FormatHandler, FormatType, ItemStatus, ValueType};
+//! Provides a generalized handler for hierarchical configuration formats 
+//! (JSON, YAML, TOML, XML).
+//!
+//! This handler works by using `serde_json::Value` as an intermediary 
+//! representation. It "flattens" nested structures into a list of 
+//! `ConfigItem`s for the TUI, and "unflattens" them back into their 
+//! nested form when saving.
+
+use super::{ConfigItem, FormatHandler, FormatType, ValueType, PathSegment};
 use serde_json::{Map, Value};
 use std::fs;
-use std::io;
 use std::path::Path;
 
+/// A format handler capable of processing nested configuration structures.
 pub struct HierarchicalHandler {
+    /// The specific file format this instance is configured to handle.
     format_type: FormatType,
 }
 
 impl HierarchicalHandler {
+    /// Creates a new hierarchical handler for the specified format.
     pub fn new(format_type: FormatType) -> Self {
         Self { format_type }
     }
 
-    fn read_value(&self, path: &Path) -> io::Result<Value> {
+    /// Reads a file and parses it into an intermediary `serde_json::Value`.
+    fn read_value(&self, path: &Path) -> anyhow::Result<Value> {
         let content = fs::read_to_string(path)?;
         let value = match self.format_type {
-            FormatType::Json => serde_json::from_str(&content)
-                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?,
-            FormatType::Yaml => serde_yaml::from_str(&content)
-                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?,
-            FormatType::Toml => toml::from_str(&content)
-                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?,
+            FormatType::Json => serde_json::from_str(&content)?,
+            FormatType::Yaml => serde_yaml::from_str(&content)?,
+            FormatType::Toml => toml::from_str(&content)?,
+            FormatType::Xml => xml_to_json(&content)?,
             _ => unreachable!(),
         };
         Ok(value)
     }
 
-    fn write_value(&self, path: &Path, value: &Value) -> io::Result<()> {
+    /// Serializes a `serde_json::Value` and writes it to the specified path.
+    fn write_value(&self, path: &Path, value: &Value) -> anyhow::Result<()> {
         let content = match self.format_type {
-            FormatType::Json => serde_json::to_string_pretty(value)
-                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?,
-            FormatType::Yaml => serde_yaml::to_string(value)
-                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?,
+            FormatType::Json => serde_json::to_string_pretty(value)?,
+            FormatType::Yaml => serde_yaml::to_string(value)?,
             FormatType::Toml => {
-                // toml requires the root to be a table
+                // TOML requires the root to be a table (Object).
                 if value.is_object() {
-                    let toml_value: toml::Value = serde_json::from_value(value.clone())
-                        .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;
-                    toml::to_string_pretty(&toml_value)
-                        .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?
+                    let toml_value: toml::Value = serde_json::from_value(value.clone())?;
+                    toml::to_string_pretty(&toml_value)?
                 } else {
-                    return Err(io::Error::new(
-                        io::ErrorKind::InvalidData,
-                        "Root of TOML must be an object",
-                    ));
+                    anyhow::bail!("Root of TOML must be an object");
                 }
             }
+            FormatType::Xml => json_to_xml(value),
             _ => unreachable!(),
         };
-        fs::write(path, content)
+        fs::write(path, content)?;
+        Ok(())
     }
 }
 
-fn flatten(value: &Value, prefix: &str, depth: usize, key_name: &str, vars: &mut Vec<ConfigItem>) {
-    let path = if prefix.is_empty() {
-        key_name.to_string()
-    } else if key_name.is_empty() {
-        prefix.to_string()
-    } else if key_name.starts_with('[') {
-        format!("{}{}", prefix, key_name)
+/// Converts an XML string into an intermediary `serde_json::Value`.
+/// 
+/// This implementation handles basic element nesting and text content. 
+/// Attributes are currently not supported.
+fn xml_to_json(content: &str) -> anyhow::Result<Value> {
+    use quick_xml::reader::Reader;
+    use quick_xml::events::Event;
+
+    let mut reader = Reader::from_str(content);
+    reader.config_mut().trim_text(true);
+    let mut buf = Vec::new();
+    
+    /// Recursively parses XML elements into a nested Map structure.
+    fn parse_recursive(reader: &mut Reader<&[u8]>) -> anyhow::Result<Value> {
+        let mut map = Map::new();
+        let mut text = String::new();
+        let mut buf = Vec::new();
+
+        loop {
+            match reader.read_event_into(&mut buf) {
+                Ok(Event::Start(e)) => {
+                    let name = String::from_utf8_lossy(e.name().as_ref()).to_string();
+                    let val = parse_recursive(reader)?;
+                    
+                    // If multiple elements have the same name, they are collected into an Array.
+                    if let Some(existing) = map.get_mut(&name) {
+                        if let Some(arr) = existing.as_array_mut() {
+                            arr.push(val);
+                        } else {
+                            let old = existing.take();
+                            *existing = Value::Array(vec![old, val]);
+                        }
+                    } else {
+                        map.insert(name, val);
+                    }
+                }
+                Ok(Event::End(_)) => break,
+                Ok(Event::Text(e)) => {
+                    text.push_str(&String::from_utf8_lossy(e.as_ref()));
+                }
+                Ok(Event::Eof) => break,
+                _ => {}
+            }
+            buf.clear();
+        }
+
+        if map.is_empty() {
+            if text.is_empty() {
+                Ok(Value::Null)
+            } else {
+                Ok(Value::String(text))
+            }
+        } else {
+            if !text.is_empty() {
+                // Special key used to store raw text content of an element that also has children.
+                map.insert("$text".to_string(), Value::String(text));
+            }
+            Ok(Value::Object(map))
+        }
+    }
+
+    // Move to the first start tag
+    loop {
+        match reader.read_event_into(&mut buf) {
+            Ok(Event::Start(e)) => {
+                let name = String::from_utf8_lossy(e.name().as_ref()).to_string();
+                let val = parse_recursive(&mut reader)?;
+                let mut root = Map::new();
+                root.insert(name, val);
+                return Ok(Value::Object(root));
+            }
+            Ok(Event::Eof) => break,
+            _ => {}
+        }
+        buf.clear();
+    }
+
+    Ok(Value::Object(Map::new()))
+}
+
+/// Converts a nested `serde_json::Value` back into an XML string.
+fn json_to_xml(value: &Value) -> String {
+    use quick_xml::Writer;
+    use quick_xml::events::{Event, BytesStart, BytesEnd, BytesText};
+
+    let mut writer = Writer::new_with_indent(Vec::new(), b' ', 4);
+    
+    /// Recursively writes JSON values as XML elements.
+    fn write_recursive(writer: &mut Writer<Vec<u8>>, value: &Value, key_name: Option<&str>) {
+        if let Some(k) = key_name
+            && k == "$text" {
+                if let Some(s) = value.as_str() {
+                    writer.write_event(Event::Text(BytesText::new(s))).unwrap();
+                }
+                return;
+            }
+        
+        match value {
+            Value::Object(map) => {
+                if let Some(k) = key_name {
+                    writer.write_event(Event::Start(BytesStart::new(k))).unwrap();
+                }
+                for (k, v) in map {
+                    if let Some(arr) = v.as_array() {
+                        for item in arr {
+                            write_recursive(writer, item, Some(k));
+                        }
+                    } else {
+                        write_recursive(writer, v, Some(k));
+                    }
+                }
+                if let Some(k) = key_name {
+                    writer.write_event(Event::End(BytesEnd::new(k))).unwrap();
+                }
+            }
+            Value::Array(arr) => {
+                for v in arr {
+                    write_recursive(writer, v, key_name);
+                }
+            }
+            Value::String(s) => {
+                if let Some(k) = key_name {
+                    writer.write_event(Event::Start(BytesStart::new(k))).unwrap();
+                }
+                writer.write_event(Event::Text(BytesText::new(s))).unwrap();
+                if let Some(k) = key_name {
+                    writer.write_event(Event::End(BytesEnd::new(k))).unwrap();
+                }
+            }
+            Value::Number(n) => {
+                if let Some(k) = key_name {
+                    writer.write_event(Event::Start(BytesStart::new(k))).unwrap();
+                }
+                writer.write_event(Event::Text(BytesText::new(&n.to_string()))).unwrap();
+                if let Some(k) = key_name {
+                    writer.write_event(Event::End(BytesEnd::new(k))).unwrap();
+                }
+            }
+            Value::Bool(b) => {
+                if let Some(k) = key_name {
+                    writer.write_event(Event::Start(BytesStart::new(k))).unwrap();
+                }
+                writer.write_event(Event::Text(BytesText::new(&b.to_string()))).unwrap();
+                if let Some(k) = key_name {
+                    writer.write_event(Event::End(BytesEnd::new(k))).unwrap();
+                }
+            }
+            Value::Null => {
+                if let Some(k) = key_name {
+                    writer.write_event(Event::Empty(BytesStart::new(k))).unwrap();
+                }
+            }
+        }
+    }
+
+    if value.is_object() {
+        for (k, v) in value.as_object().unwrap() {
+            if let Some(arr) = v.as_array() {
+                for item in arr {
+                    write_recursive(&mut writer, item, Some(k));
+                }
+            } else {
+                write_recursive(&mut writer, v, Some(k));
+            }
+        }
     } else {
-        format!("{}.{}", prefix, key_name)
+        write_recursive(&mut writer, value, None);
+    }
+    
+    let mut out = String::from_utf8(writer.into_inner()).unwrap();
+    if !out.ends_with('\n') {
+        out.push('\n');
+    }
+    out
+}
+
+/// Recursively flattens a nested `serde_json::Value` into a list of `ConfigItem`s.
+///
+/// This function translates the structural hierarchy of the JSON into 
+/// linear entries with associated paths and depth levels for UI rendering.
+fn flatten(value: &Value, current_path: Vec<PathSegment>, key_name: Option<String>, depth: usize, vars: &mut Vec<ConfigItem>) {
+    let mut next_path = current_path.clone();
+    
+    if let Some(ref k) = key_name {
+        if !current_path.is_empty() {
+            // It's a key in an object, so append to path
+            next_path.push(PathSegment::Key(k.clone()));
+        } else {
+            // First element, maybe root
+            if !k.is_empty() {
+                next_path.push(PathSegment::Key(k.clone()));
+            }
+        }
+    }
+
+    let display_key = match next_path.last() {
+        Some(PathSegment::Key(k)) => k.clone(),
+        Some(PathSegment::Index(i)) => format!("[{}]", i),
+        None => "".to_string(),
     };
 
     match value {
         Value::Object(map) => {
-            if !path.is_empty() {
-                vars.push(ConfigItem {
-                    key: key_name.to_string(),
-                    path: path.clone(),
-                    value: None,
-                    template_value: None,
-                    default_value: None,
-                    depth,
-                    is_group: true,
-                    status: ItemStatus::Present,
-                    value_type: ValueType::Null,
-                });
+            if !next_path.is_empty() {
+                vars.push(ConfigItem::group(display_key, next_path.clone(), depth));
             }
-            let next_depth = if path.is_empty() { depth } else { depth + 1 };
+            let next_depth = if next_path.is_empty() { depth } else { depth + 1 };
             for (k, v) in map {
-                flatten(v, &path, next_depth, k, vars);
+                flatten(v, next_path.clone(), Some(k.clone()), next_depth, vars);
             }
         }
         Value::Array(arr) => {
-            if !path.is_empty() {
-                vars.push(ConfigItem {
-                    key: key_name.to_string(),
-                    path: path.clone(),
-                    value: None,
-                    template_value: None,
-                    default_value: None,
-                    depth,
-                    is_group: true,
-                    status: ItemStatus::Present,
-                    value_type: ValueType::Null,
-                });
+            if !next_path.is_empty() {
+                vars.push(ConfigItem::group(display_key, next_path.clone(), depth));
             }
-            let next_depth = if path.is_empty() { depth } else { depth + 1 };
+            let next_depth = if next_path.is_empty() { depth } else { depth + 1 };
             for (i, v) in arr.iter().enumerate() {
-                let array_key = format!("[{}]", i);
-                flatten(v, &path, next_depth, &array_key, vars);
+                let mut arr_path = next_path.clone();
+                arr_path.push(PathSegment::Index(i));
+                flatten(v, arr_path, None, next_depth, vars);
             }
         }
         Value::String(s) => {
-            vars.push(ConfigItem {
-                key: key_name.to_string(),
-                path: path.clone(),
-                value: Some(s.clone()),
-                template_value: Some(s.clone()),
-                default_value: Some(s.clone()),
-                depth,
-                is_group: false,
-                status: ItemStatus::Present,
-                value_type: ValueType::String,
-            });
+            vars.push(ConfigItem::leaf(display_key, next_path.clone(), s.clone(), depth, ValueType::String));
         }
         Value::Number(n) => {
             let s = n.to_string();
-            vars.push(ConfigItem {
-                key: key_name.to_string(),
-                path: path.clone(),
-                value: Some(s.clone()),
-                template_value: Some(s.clone()),
-                default_value: Some(s.clone()),
-                depth,
-                is_group: false,
-                status: ItemStatus::Present,
-                value_type: ValueType::Number,
-            });
+            vars.push(ConfigItem::leaf(display_key, next_path.clone(), s.clone(), depth, ValueType::Number));
         }
         Value::Bool(b) => {
             let s = b.to_string();
-            vars.push(ConfigItem {
-                key: key_name.to_string(),
-                path: path.clone(),
-                value: Some(s.clone()),
-                template_value: Some(s.clone()),
-                default_value: Some(s.clone()),
-                depth,
-                is_group: false,
-                status: ItemStatus::Present,
-                value_type: ValueType::Bool,
-            });
+            vars.push(ConfigItem::leaf(display_key, next_path.clone(), s.clone(), depth, ValueType::Bool));
         }
         Value::Null => {
-            vars.push(ConfigItem {
-                key: key_name.to_string(),
-                path: path.clone(),
-                value: Some("".to_string()),
-                template_value: Some("".to_string()),
-                default_value: Some("".to_string()),
-                depth,
-                is_group: false,
-                status: ItemStatus::Present,
-                value_type: ValueType::Null,
-            });
+            vars.push(ConfigItem::leaf(display_key, next_path.clone(), "", depth, ValueType::Null));
         }
     }
 }
 
 impl FormatHandler for HierarchicalHandler {
-    fn parse(&self, path: &Path) -> io::Result<Vec<ConfigItem>> {
+    fn parse(&self, path: &Path) -> anyhow::Result<Vec<ConfigItem>> {
         let value = self.read_value(path)?;
         let mut vars = Vec::new();
-        flatten(&value, "", 0, "", &mut vars);
+        flatten(&value, Vec::new(), Some("".to_string()), 0, &mut vars);
         Ok(vars)
     }
 
-    fn merge(&self, path: &Path, vars: &mut Vec<ConfigItem>) -> io::Result<()> {
-        if !path.exists() {
-            return Ok(());
-        }
-        let existing_value = self.read_value(path)?;
-        let mut existing_vars = Vec::new();
-        flatten(&existing_value, "", 0, "", &mut existing_vars);
-
-        for var in vars.iter_mut() {
-            if let Some(existing) = existing_vars.iter().find(|v| v.path == var.path) {
-                if var.value != existing.value {
-                    var.value = existing.value.clone();
-                    var.status = ItemStatus::Modified;
-                }
-            } else {
-                var.status = ItemStatus::MissingFromActive;
-            }
-        }
-        
-        // Find keys in active that are not in template
-        let mut to_add = Vec::new();
-        for existing in existing_vars {
-            if !vars.iter().any(|v| v.path == existing.path) {
-                let mut new_item = existing.clone();
-                new_item.status = ItemStatus::MissingFromTemplate;
-                new_item.template_value = None;
-                new_item.default_value = None;
-                to_add.push(new_item);
-            }
-        }
-        
-        // Basic insertion logic for extra keys (could be improved to insert at correct depth/position)
-        vars.extend(to_add);
-
-        Ok(())
-    }
-
-    fn write(&self, path: &Path, vars: &[ConfigItem]) -> io::Result<()> {
+    fn write(&self, path: &Path, vars: &[ConfigItem]) -> anyhow::Result<()> {
         let mut root = Value::Object(Map::new());
         for var in vars {
-            if !var.is_group {
-                let val = var.value.as_deref()
-                    .or(var.template_value.as_deref())
-                    .unwrap_or("");
-                insert_into_value(&mut root, &var.path, val, var.value_type);
+            if var.should_write() {
+                insert_into_value(&mut root, &var.path, var.write_value(), var.value_type);
             }
         }
         self.write_value(path, &root)
     }
 }
 
-fn insert_into_value(root: &mut Value, path: &str, new_val_str: &str, value_type: ValueType) {
-    let mut parts = path.split('.');
-    let last_part = match parts.next_back() {
-        Some(p) => p,
-        None => return,
-    };
+/// Reconstructs a nested `serde_json::Value` from a hierarchical path and a leaf value.
+/// 
+/// This is the core "unflattening" logic used when saving modified configuration.
+fn insert_into_value(root: &mut Value, path: &[PathSegment], new_val_str: &str, value_type: ValueType) {
+    if path.is_empty() {
+        return;
+    }
 
     let mut current = root;
-    for part in parts {
-        let (key, idx) = parse_array_key(part);
-        if !current.is_object() {
-            *current = Value::Object(Map::new());
-        }
-        let map = current.as_object_mut().unwrap();
+    
+    // Traverse all but the last segment to build the intermediate structure.
+    for i in 0..path.len() - 1 {
+        let segment = &path[i];
+        let next_segment = &path[i + 1];
 
-        let next_node = map.entry(key.to_string()).or_insert_with(|| {
-            if idx.is_some() {
-                Value::Array(Vec::new())
-            } else {
-                Value::Object(Map::new())
+        match segment {
+            PathSegment::Key(key) => {
+                if !current.is_object() {
+                    *current = Value::Object(Map::new());
+                }
+                let map = current.as_object_mut().unwrap();
+                
+                let next_node = map.entry(key.clone()).or_insert_with(|| {
+                    match next_segment {
+                        PathSegment::Index(_) => Value::Array(Vec::new()),
+                        PathSegment::Key(_) => Value::Object(Map::new()),
+                    }
+                });
+                current = next_node;
             }
-        });
-
-        if let Some(i) = idx {
-            if !next_node.is_array() {
-                *next_node = Value::Array(Vec::new());
+            PathSegment::Index(idx) => {
+                if !current.is_array() {
+                    *current = Value::Array(Vec::new());
+                }
+                let arr = current.as_array_mut().unwrap();
+                while arr.len() <= *idx {
+                    match next_segment {
+                        PathSegment::Index(_) => arr.push(Value::Array(Vec::new())),
+                        PathSegment::Key(_) => arr.push(Value::Object(Map::new())),
+                    }
+                }
+                current = &mut arr[*idx];
             }
-            let arr = next_node.as_array_mut().unwrap();
-            while arr.len() <= i {
-                arr.push(Value::Object(Map::new()));
-            }
-            current = &mut arr[i];
-        } else {
-            current = next_node;
         }
     }
 
-    let (final_key, final_idx) = parse_array_key(last_part);
-    if !current.is_object() {
-        *current = Value::Object(Map::new());
-    }
-    let map = current.as_object_mut().unwrap();
-
-    // Use the preserved ValueType instead of aggressive inference
+    // Insert the actual leaf value at the end of the path.
+    let final_segment = &path[path.len() - 1];
+    
+    // Attempt to preserve the original data type during insertion.
     let final_val = match value_type {
         ValueType::Number => {
             if let Ok(n) = new_val_str.parse::<i64>() {
@@ -289,40 +391,34 @@ fn insert_into_value(root: &mut Value, path: &str, new_val_str: &str, value_type
         _ => Value::String(new_val_str.to_string()),
     };
 
-    if let Some(i) = final_idx {
-        let next_node = map
-            .entry(final_key.to_string())
-            .or_insert_with(|| Value::Array(Vec::new()));
-        if !next_node.is_array() {
-            *next_node = Value::Array(Vec::new());
+    match final_segment {
+        PathSegment::Key(key) => {
+            if !current.is_object() {
+                *current = Value::Object(Map::new());
+            }
+            let map = current.as_object_mut().unwrap();
+            map.insert(key.clone(), final_val);
         }
-        let arr = next_node.as_array_mut().unwrap();
-        while arr.len() <= i {
-            arr.push(Value::Null);
+        PathSegment::Index(idx) => {
+            if !current.is_array() {
+                *current = Value::Array(Vec::new());
+            }
+            let arr = current.as_array_mut().unwrap();
+            while arr.len() <= *idx {
+                arr.push(Value::Null);
+            }
+            arr[*idx] = final_val;
         }
-        arr[i] = final_val;
-    } else {
-        map.insert(final_key.to_string(), final_val);
-    }
-}
-
-fn parse_array_key(part: &str) -> (&str, Option<usize>) {
-    if part.ends_with(']') && part.contains('[') {
-        let start_idx = part.find('[').unwrap();
-        let key = &part[..start_idx];
-        let idx = part[start_idx + 1..part.len() - 1].parse::<usize>().ok();
-        (key, idx)
-    } else {
-        (part, None)
     }
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::format::ItemStatus;
 
     #[test]
-    fn test_flatten_unflatten() {
+    fn test_json_flatten_unflatten() {
         let mut vars = Vec::new();
         let json = serde_json::json!({
             "services": {
@@ -335,7 +431,7 @@ mod tests {
             }
         });
 
-        flatten(&json, "", 0, "", &mut vars);
+        flatten(&json, Vec::new(), Some("".to_string()), 0, &mut vars);
         assert_eq!(vars.len(), 6);
 
         let mut root = Value::Object(Map::new());
@@ -345,9 +441,207 @@ mod tests {
             }
         }
 
-        // When unflattening, it parses bool back
         let unflattened_json = serde_json::to_string(&root).unwrap();
         assert!(unflattened_json.contains("\"8080:80\""));
         assert!(unflattened_json.contains("true"));
     }
+
+    #[test]
+    fn test_type_preservation() {
+        let mut vars = Vec::new();
+        let json = serde_json::json!({
+            "port_num": 8080,
+            "port_str": "8080",
+            "is_enabled": true,
+            "is_enabled_str": "true",
+            "float_num": 42.42,
+            "float_str": "42.42"
+        });
+
+        flatten(&json, Vec::new(), Some("".to_string()), 0, &mut vars);
+        
+        let mut root = Value::Object(Map::new());
+        for var in vars {
+            if !var.is_group {
+                insert_into_value(&mut root, &var.path, var.value.as_deref().unwrap_or(""), var.value_type);
+            }
+        }
+
+        let unflattened = root.as_object().unwrap();
+        
+        assert!(unflattened["port_num"].is_number(), "port_num should be a number");
+        assert_eq!(unflattened["port_num"].as_i64(), Some(8080));
+        
+        assert!(unflattened["port_str"].is_string(), "port_str should be a string");
+        assert_eq!(unflattened["port_str"].as_str(), Some("8080"));
+        
+        assert!(unflattened["is_enabled"].is_boolean(), "is_enabled should be a boolean");
+        assert_eq!(unflattened["is_enabled"].as_bool(), Some(true));
+        
+        assert!(unflattened["is_enabled_str"].is_string(), "is_enabled_str should be a string");
+        assert_eq!(unflattened["is_enabled_str"].as_str(), Some("true"));
+        
+        assert!(unflattened["float_num"].is_number(), "float_num should be a number");
+        assert_eq!(unflattened["float_num"].as_f64(), Some(42.42));
+        
+        assert!(unflattened["float_str"].is_string(), "float_str should be a string");
+        assert_eq!(unflattened["float_str"].as_str(), Some("42.42"));
+    }
+
+    #[test]
+    fn test_yaml_flatten_unflatten() {
+        let yaml_str = "
+server:
+  port: 8080
+  port_str: \"8080\"
+  enabled: true
+";
+        let yaml_val: Value = serde_yaml::from_str(yaml_str).unwrap();
+        let mut vars = Vec::new();
+        flatten(&yaml_val, Vec::new(), Some("".to_string()), 0, &mut vars);
+        
+        let mut root = Value::Object(Map::new());
+        for var in vars {
+            if !var.is_group {
+                insert_into_value(&mut root, &var.path, var.value.as_deref().unwrap_or(""), var.value_type);
+            }
+        }
+        
+        let unflattened_yaml = serde_yaml::to_string(&root).unwrap();
+        assert!(unflattened_yaml.contains("port: 8080"));
+        assert!(unflattened_yaml.contains("port_str: '8080'") || unflattened_yaml.contains("port_str: \"8080\""));
+        assert!(unflattened_yaml.contains("enabled: true"));
+    }
+
+    #[test]
+    fn test_toml_flatten_unflatten() {
+        let toml_str = "
+[server]
+port = 8080
+port_str = \"8080\"
+enabled = true
+";
+        let toml_val: toml::Value = toml::from_str(toml_str).unwrap();
+        let json_val: Value = serde_json::to_value(toml_val).unwrap();
+
+        let mut vars = Vec::new();
+        flatten(&json_val, Vec::new(), Some("".to_string()), 0, &mut vars);
+        
+        let mut root = Value::Object(Map::new());
+        for var in vars {
+            if !var.is_group {
+                insert_into_value(&mut root, &var.path, var.value.as_deref().unwrap_or(""), var.value_type);
+            }
+        }
+        
+        let toml_root: toml::Value = serde_json::from_value(root).unwrap();
+        let unflattened_toml = toml::to_string(&toml_root).unwrap();
+        
+        assert!(unflattened_toml.contains("port = 8080"));
+        assert!(unflattened_toml.contains("port_str = \"8080\""));
+        assert!(unflattened_toml.contains("enabled = true"));
+    }
+
+    #[test]
+    fn test_group_rename_write() {
+        let mut vars = Vec::new();
+        let json = serde_json::json!({
+            "old_group": {
+                "key": "val"
+            }
+        });
+
+        flatten(&json, Vec::new(), Some("".to_string()), 0, &mut vars);
+        assert_eq!(vars.len(), 2);
+        assert_eq!(vars[0].key, "old_group");
+        assert!(vars[0].is_group);
+        assert_eq!(vars[1].key, "key");
+        assert_eq!(vars[1].path_string(), "old_group.key");
+
+        // Manually simulate a rename of "old_group" to "new_group"
+        vars[0].key = "new_group".to_string();
+        vars[0].path = vec![PathSegment::Key("new_group".to_string())];
+        vars[1].path = vec![
+            PathSegment::Key("new_group".to_string()),
+            PathSegment::Key("key".to_string()),
+        ];
+
+        let mut root = Value::Object(Map::new());
+        for var in &vars {
+            if !var.is_group {
+                insert_into_value(&mut root, &var.path, var.value.as_deref().unwrap_or(""), var.value_type);
+            }
+        }
+        
+        let out = serde_json::to_string(&root).unwrap();
+        assert!(out.contains("\"new_group\""));
+        assert!(out.contains("\"key\":\"val\""));
+        assert!(!out.contains("\"old_group\""));
+    }
+
+    #[test]
+    fn missing_nested_keys_are_spliced_beside_their_siblings() {
+        use std::io::Write;
+        let handler = HierarchicalHandler::new(FormatType::Json);
+
+        let mut template = tempfile::NamedTempFile::new().unwrap();
+        write!(
+            template,
+            r#"{{"server":{{"host":"localhost","port":8080}},"debug":false}}"#
+        )
+        .unwrap();
+
+        // The active config is missing `server.port` entirely.
+        let mut active = Vec::new();
+        flatten(
+            &serde_json::json!({"server": {"host": "example.com"}, "debug": true}),
+            Vec::new(),
+            Some("".to_string()),
+            0,
+            &mut active,
+        );
+
+        handler.merge(template.path(), &mut active).unwrap();
+
+        let order: Vec<String> = active.iter().map(|v| v.path_string()).collect();
+        assert_eq!(order, vec!["server", "server.host", "server.port", "debug"]);
+
+        let port = active.iter().find(|v| v.key == "port").unwrap();
+        assert_eq!(port.status, ItemStatus::MissingFromActive);
+        assert_eq!(port.depth, 1);
+        assert_eq!(port.template_value.as_deref(), Some("8080"));
+    }
+
+    #[test]
+    fn a_malformed_file_is_an_error_not_an_empty_document() {
+        use std::io::Write;
+        let handler = HierarchicalHandler::new(FormatType::Json);
+        let mut file = tempfile::NamedTempFile::new().unwrap();
+        write!(file, "{{ not json").unwrap();
+
+        assert!(handler.parse(file.path()).is_err());
+    }
+
+    #[test]
+    fn unaccepted_template_entries_are_not_written() {
+        let handler = HierarchicalHandler::new(FormatType::Json);
+        let mut vars = Vec::new();
+        flatten(
+            &serde_json::json!({"kept": "yes", "skipped": "no"}),
+            Vec::new(),
+            Some("".to_string()),
+            0,
+            &mut vars,
+        );
+        let skipped = vars.iter_mut().find(|v| v.key == "skipped").unwrap();
+        skipped.status = ItemStatus::MissingFromActive;
+        skipped.template_value = skipped.value.take();
+
+        let file = tempfile::NamedTempFile::new().unwrap();
+        handler.write(file.path(), &vars).unwrap();
+
+        let content = std::fs::read_to_string(file.path()).unwrap();
+        assert!(content.contains("kept"));
+        assert!(!content.contains("skipped"));
+    }
 }

src/format/ini.rs

@@ -0,0 +1,98 @@
+use super::{ConfigItem, FormatHandler, ValueType, PathSegment};
+use ini::Ini;
+use std::path::Path;
+
+pub struct IniHandler;
+
+impl FormatHandler for IniHandler {
+    fn parse(&self, path: &Path) -> anyhow::Result<Vec<ConfigItem>> {
+        let conf = Ini::load_from_file(path)?;
+        let mut vars = Vec::new();
+
+        for (section, prop) in &conf {
+            let section_name = section.unwrap_or_default();
+            
+            if !section_name.is_empty() {
+                vars.push(ConfigItem::group(
+                    section_name,
+                    vec![PathSegment::Key(section_name.to_string())],
+                    0,
+                ));
+            }
+
+            for (key, value) in prop {
+                let path = if section_name.is_empty() {
+                    vec![PathSegment::Key(key.to_string())]
+                } else {
+                    vec![PathSegment::Key(section_name.to_string()), PathSegment::Key(key.to_string())]
+                };
+
+                vars.push(ConfigItem::leaf(
+                    key,
+                    path,
+                    value,
+                    if section_name.is_empty() { 0 } else { 1 },
+                    ValueType::String,
+                ));
+            }
+        }
+
+        Ok(vars)
+    }
+
+    fn write(&self, path: &Path, vars: &[ConfigItem]) -> anyhow::Result<()> {
+        let mut conf = Ini::new();
+        for var in vars {
+            if var.should_write() {
+                let val = var.write_value();
+
+                if var.path.len() == 2 {
+                    if let (PathSegment::Key(section), PathSegment::Key(key)) = (&var.path[0], &var.path[1]) {
+                        conf.with_section(Some(section)).set(key, val);
+                    }
+                } else if var.path.len() == 1
+                    && let PathSegment::Key(key) = &var.path[0] {
+                        conf.with_section(None::<String>).set(key, val);
+                    }
+            }
+        }
+        conf.write_to_file(path)?;
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tempfile::NamedTempFile;
+
+    #[test]
+    fn test_section_rename_write() {
+        let handler = IniHandler;
+        let mut vars = vec![
+            ConfigItem::group("server", vec![PathSegment::Key("server".to_string())], 0),
+            ConfigItem::leaf(
+                "port",
+                vec![PathSegment::Key("server".to_string()), PathSegment::Key("port".to_string())],
+                "8080",
+                1,
+                ValueType::String,
+            ),
+        ];
+
+        // Rename "server" to "srv"
+        vars[0].key = "srv".to_string();
+        vars[0].path = vec![PathSegment::Key("srv".to_string())];
+        
+        // Update child path
+        vars[1].path = vec![PathSegment::Key("srv".to_string()), PathSegment::Key("port".to_string())];
+        
+        let file = NamedTempFile::new().unwrap();
+        handler.write(file.path(), &vars).unwrap();
+        
+        let content = std::fs::read_to_string(file.path()).unwrap();
+        assert!(content.contains("[srv]"));
+        assert!(content.contains("port=8080"));
+        assert!(!content.contains("[server]"));
+    }
+}

src/format/mod.rs

@@ -1,52 +1,273 @@
-use std::io;
+//! This module defines the unified data model used by `mould` to represent 
+//! configuration data across all supported file formats.
+//!
+//! By normalizing heterogeneous structures (like nested YAML or flat .env) 
+//! into a standard tree-like representation, the TUI logic remains 
+//! independent of the underlying file format.
+
 use std::path::Path;
 
 pub mod env;
 pub mod hierarchical;
+pub mod ini;
+pub mod properties;
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+/// Represents the status of a configuration item relative to its template.
+///
+/// This describes the *template comparison* only. Whether the user has edited
+/// an item during this session is tracked separately by [`ConfigItem::dirty`],
+/// so that a config which legitimately differs from its example file does not
+/// render as if every line had just been touched.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum ItemStatus {
+    /// Item exists in the active configuration and matches the template (or no template exists).
     Present,
+    /// Item exists in the template but is missing from the active configuration.
     MissingFromActive,
-    MissingFromTemplate,
-    Modified,
+    /// Item exists in both, but the active value differs from the template default.
+    Overridden,
 }
 
+/// Hints about the original data type to ensure correct serialization during writes.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum ValueType {
+    /// Standard text.
     String,
+    /// Numeric values (integers or floats).
     Number,
+    /// True/False values.
     Bool,
+    /// Representing an explicit null or empty value.
     Null,
 }
 
-#[derive(Debug, Clone)]
-pub struct ConfigItem {
-    pub key: String,
-    pub path: String,
-    pub value: Option<String>,
-    pub template_value: Option<String>,
-    pub default_value: Option<String>,
-    pub depth: usize,
-    pub is_group: bool,
-    pub status: ItemStatus,
-    pub value_type: ValueType,
+/// A single segment in a hierarchical configuration path.
+///
+/// For example, `services[0].image` would be represented as:
+/// `[Key("services"), Index(0), Key("image")]`
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum PathSegment {
+    /// A named key in an object/map.
+    Key(String),
+    /// A numeric index in an array/list.
+    Index(usize),
 }
 
+impl std::fmt::Display for PathSegment {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            PathSegment::Key(k) => write!(f, "{}", k),
+            PathSegment::Index(i) => write!(f, "[{}]", i),
+        }
+    }
+}
+
+/// The unified representation of a single configuration entry.
+///
+/// This model is used for UI rendering and internal manipulation. 
+/// Format-specific handlers are responsible for translating their native 
+/// data into this structure.
+#[derive(Debug, Clone)]
+pub struct ConfigItem {
+    /// The short display name of the key (e.g., `port`).
+    pub key: String,
+    /// The full hierarchical path defining this item's location in the config tree.
+    pub path: Vec<PathSegment>,
+    /// The active value of the configuration entry.
+    pub value: Option<String>,
+    /// The value found in the template file (if any).
+    pub template_value: Option<String>,
+    /// A fallback value to use if the item is missing.
+    pub default_value: Option<String>,
+    /// Visual depth in the tree (used for indentation in the TUI).
+    pub depth: usize,
+    /// True if this item represents a structural node (object or array) rather than a leaf value.
+    pub is_group: bool,
+    /// Comparison status relative to the template.
+    pub status: ItemStatus,
+    /// Metadata about the original data type.
+    pub value_type: ValueType,
+    /// True if the user changed this item during the current session.
+    pub dirty: bool,
+    /// True if this group's children are folded away in the tree view.
+    pub collapsed: bool,
+    /// Comment and blank lines that preceded this entry in the source file.
+    ///
+    /// Replayed verbatim on write by formats that carry inline documentation
+    /// (currently `.env`), so that annotated templates survive a save.
+    pub leading_trivia: Vec<String>,
+}
+
+impl Default for ConfigItem {
+    fn default() -> Self {
+        Self {
+            key: String::new(),
+            path: Vec::new(),
+            value: None,
+            template_value: None,
+            default_value: None,
+            depth: 0,
+            is_group: false,
+            status: ItemStatus::Present,
+            value_type: ValueType::String,
+            dirty: false,
+            collapsed: false,
+            leading_trivia: Vec::new(),
+        }
+    }
+}
+
+impl ConfigItem {
+    /// Builds a structural node (object, array, or section).
+    pub fn group(key: impl Into<String>, path: Vec<PathSegment>, depth: usize) -> Self {
+        Self {
+            key: key.into(),
+            path,
+            depth,
+            is_group: true,
+            value_type: ValueType::Null,
+            ..Default::default()
+        }
+    }
+
+    /// Builds a leaf entry holding an actual value.
+    ///
+    /// `template_value` and `default_value` are intentionally left unset: they are
+    /// populated by [`FormatHandler::merge`] from the template file, and mean
+    /// nothing when a file is parsed on its own.
+    pub fn leaf(
+        key: impl Into<String>,
+        path: Vec<PathSegment>,
+        value: impl Into<String>,
+        depth: usize,
+        value_type: ValueType,
+    ) -> Self {
+        Self {
+            key: key.into(),
+            path,
+            value: Some(value.into()),
+            depth,
+            value_type,
+            ..Default::default()
+        }
+    }
+
+    /// Returns a human-readable string representation of the full path (e.g., `server.port`).
+    pub fn path_string(&self) -> String {
+        let mut s = String::new();
+        for (i, segment) in self.path.iter().enumerate() {
+            match segment {
+                PathSegment::Key(k) => {
+                    if i > 0 {
+                        s.push('.');
+                    }
+                    s.push_str(k);
+                }
+                PathSegment::Index(idx) => {
+                    s.push_str(&format!("[{}]", idx));
+                }
+            }
+        }
+        s
+    }
+
+    /// True if this entry belongs in the output file.
+    ///
+    /// Items that exist only in the template and have not been accepted by the
+    /// user are deliberately omitted, which is what makes the "(missing)" marker
+    /// mean something: nothing is written until it is pulled in explicitly.
+    pub fn should_write(&self) -> bool {
+        !self.is_group && self.status != ItemStatus::MissingFromActive
+    }
+
+    /// The string that should be serialized for this entry.
+    pub fn write_value(&self) -> &str {
+        self.value.as_deref().unwrap_or("")
+    }
+
+    /// True if this item sits inside the subtree rooted at `prefix` (and is not `prefix` itself).
+    pub fn is_descendant_of(&self, prefix: &[PathSegment]) -> bool {
+        self.path.len() > prefix.len() && self.path.starts_with(prefix)
+    }
+}
+
+/// Supported configuration file formats.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum FormatType {
     Env,
     Json,
     Yaml,
     Toml,
+    Xml,
+    Ini,
+    Properties,
 }
 
+/// Defines the interface for parsing, merging, and writing configuration files.
+///
+/// Implementing this trait allows `mould` to support new file formats.
 pub trait FormatHandler {
-    fn parse(&self, path: &Path) -> io::Result<Vec<ConfigItem>>;
-    fn merge(&self, path: &Path, vars: &mut Vec<ConfigItem>) -> io::Result<()>;
-    fn write(&self, path: &Path, vars: &[ConfigItem]) -> io::Result<()>;
+    /// Parses a file into the unified `Vec<ConfigItem>` representation.
+    fn parse(&self, path: &Path) -> anyhow::Result<Vec<ConfigItem>>;
+
+    /// Merges an active configuration with a template file.
+    ///
+    /// This identifies missing keys, records template defaults, and splices
+    /// template-only entries in at their natural position so that the tree view
+    /// keeps reflecting the real structure.
+    fn merge(&self, path: &Path, vars: &mut Vec<ConfigItem>) -> anyhow::Result<()> {
+        if !path.exists() {
+            return Ok(());
+        }
+
+        let template_vars = self.parse(path)?;
+
+        for var in vars.iter_mut() {
+            if let Some(template_var) = template_vars.iter().find(|v| v.path == var.path) {
+                var.template_value = template_var.value.clone();
+                var.default_value = template_var.value.clone();
+
+                var.status = if var.value == template_var.value {
+                    ItemStatus::Present
+                } else {
+                    ItemStatus::Overridden
+                };
+            } else {
+                // Exists in active, but not in template.
+                var.status = ItemStatus::Present;
+            }
+        }
+
+        // Splice in template-only entries. `template_vars` is in document order
+        // (a pre-order walk of the tree), so tracking an anchor as we go keeps
+        // each missing entry next to the siblings it belongs with instead of
+        // stranding it at the end of the list.
+        let mut anchor = 0usize;
+        for template_var in &template_vars {
+            if let Some(pos) = vars.iter().position(|v| v.path == template_var.path) {
+                anchor = pos + 1;
+                continue;
+            }
+
+            let mut new_item = template_var.clone();
+            new_item.status = ItemStatus::MissingFromActive;
+            new_item.template_value = template_var.value.clone();
+            new_item.default_value = template_var.value.clone();
+            new_item.value = None;
+
+            let at = anchor.min(vars.len());
+            vars.insert(at, new_item);
+            anchor = at + 1;
+        }
+
+        Ok(())
+    }
+
+    /// Writes the unified representation back to the original file format.
+    fn write(&self, path: &Path, vars: &[ConfigItem]) -> anyhow::Result<()>;
 }
 
+/// Automatically detects the configuration format based on file extension or an explicit override.
 pub fn detect_format(path: &Path, override_format: Option<String>) -> FormatType {
     if let Some(fmt) = override_format {
         match fmt.to_lowercase().as_str() {
@@ -54,27 +275,34 @@ pub fn detect_format(path: &Path, override_format: Option<String>) -> FormatType
             "json" => return FormatType::Json,
             "yaml" | "yml" => return FormatType::Yaml,
             "toml" => return FormatType::Toml,
+            "xml" => return FormatType::Xml,
+            "ini" => return FormatType::Ini,
+            "properties" => return FormatType::Properties,
             _ => {}
         }
     }
 
-    let file_name = path.file_name().unwrap_or_default().to_string_lossy();
-    if file_name.ends_with(".json") {
-        FormatType::Json
-    } else if file_name.ends_with(".yaml") || file_name.ends_with(".yml") {
-        FormatType::Yaml
-    } else if file_name.ends_with(".toml") {
-        FormatType::Toml
-    } else {
-        FormatType::Env
+    let ext = path.extension().and_then(|s| s.to_str()).unwrap_or_default();
+    match ext {
+        "json" => FormatType::Json,
+        "yaml" | "yml" => FormatType::Yaml,
+        "toml" => FormatType::Toml,
+        "xml" => FormatType::Xml,
+        "ini" => FormatType::Ini,
+        "properties" => FormatType::Properties,
+        _ => FormatType::Env,
     }
 }
 
+/// Factory function to return the appropriate handler implementation for a given format.
 pub fn get_handler(format: FormatType) -> Box<dyn FormatHandler> {
     match format {
         FormatType::Env => Box::new(env::EnvHandler),
         FormatType::Json => Box::new(hierarchical::HierarchicalHandler::new(FormatType::Json)),
         FormatType::Yaml => Box::new(hierarchical::HierarchicalHandler::new(FormatType::Yaml)),
         FormatType::Toml => Box::new(hierarchical::HierarchicalHandler::new(FormatType::Toml)),
+        FormatType::Xml => Box::new(hierarchical::HierarchicalHandler::new(FormatType::Xml)),
+        FormatType::Ini => Box::new(ini::IniHandler),
+        FormatType::Properties => Box::new(properties::PropertiesHandler),
     }
 }

src/format/properties.rs

@@ -0,0 +1,101 @@
+use super::{ConfigItem, FormatHandler, ValueType, PathSegment};
+use java_properties::{LineContent, PropertiesIter, PropertiesWriter};
+use std::fs::File;
+use std::io::{BufReader, BufWriter};
+use std::path::Path;
+
+pub struct PropertiesHandler;
+
+impl FormatHandler for PropertiesHandler {
+    fn parse(&self, path: &Path) -> anyhow::Result<Vec<ConfigItem>> {
+        let file = File::open(path)?;
+        let reader = BufReader::new(file);
+        let iter = PropertiesIter::new(reader);
+
+        let mut vars = Vec::new();
+        let mut groups = std::collections::HashSet::new();
+
+        for line_result in iter {
+            let line = line_result?;
+            
+            if let LineContent::KVPair(path, value) = line.consume_content() {
+                // Add groups based on dot notation
+                let parts: Vec<&str> = path.split('.').collect();
+                let mut current_path = Vec::new();
+                
+                for (i, part) in parts.iter().enumerate().take(parts.len().saturating_sub(1)) {
+                    current_path.push(PathSegment::Key(part.to_string()));
+                    
+                    if groups.insert(current_path.clone()) {
+                        vars.push(ConfigItem::group(*part, current_path.clone(), i));
+                    }
+                }
+
+                let mut final_path = current_path.clone();
+                let last_key = parts.last().unwrap_or(&"").to_string();
+                final_path.push(PathSegment::Key(last_key.clone()));
+
+                vars.push(ConfigItem::leaf(
+                    last_key,
+                    final_path,
+                    value,
+                    parts.len().saturating_sub(1),
+                    ValueType::String,
+                ));
+            }
+        }
+
+
+        Ok(vars)
+    }
+
+    fn write(&self, path: &Path, vars: &[ConfigItem]) -> anyhow::Result<()> {
+        let file = File::create(path)?;
+        let writer = BufWriter::new(file);
+        let mut prop_writer = PropertiesWriter::new(writer);
+        
+        for var in vars {
+            if var.should_write() {
+                prop_writer.write(&var.path_string(), var.write_value())?;
+            }
+        }
+
+        prop_writer.finish()?;
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tempfile::NamedTempFile;
+
+    #[test]
+    fn test_group_rename_write() {
+        let handler = PropertiesHandler;
+        let mut vars = vec![
+            ConfigItem::group("server", vec![PathSegment::Key("server".to_string())], 0),
+            ConfigItem::leaf(
+                "port",
+                vec![PathSegment::Key("server".to_string()), PathSegment::Key("port".to_string())],
+                "8080",
+                1,
+                ValueType::String,
+            ),
+        ];
+
+        // Rename "server" to "srv"
+        vars[0].key = "srv".to_string();
+        vars[0].path = vec![PathSegment::Key("srv".to_string())];
+        
+        // Update child path
+        vars[1].path = vec![PathSegment::Key("srv".to_string()), PathSegment::Key("port".to_string())];
+        
+        let file = NamedTempFile::new().unwrap();
+        handler.write(file.path(), &vars).unwrap();
+        
+        let content = std::fs::read_to_string(file.path()).unwrap();
+        assert!(content.contains("srv.port=8080"));
+        assert!(!content.contains("server.port=8080"));
+    }
+}

src/main.rs

@@ -5,6 +5,8 @@ mod error;
 mod format;
 mod runner;
 mod ui;
+mod resolver;
+mod undo;
 
 use app::App;
 use config::load_config;
@@ -13,123 +15,14 @@ use format::{detect_format, get_handler};
 use log::{error, info, warn};
 use runner::AppRunner;
 use std::io;
-use std::path::{Path, PathBuf};
 
+use anyhow::Context;
 use crossterm::{
-    event::{DisableMouseCapture, EnableMouseCapture},
     execute,
     terminal::{EnterAlternateScreen, LeaveAlternateScreen, disable_raw_mode, enable_raw_mode},
 };
 use ratatui::{Terminal, backend::CrosstermBackend};
 
-/// Helper to automatically determine the output file path based on common naming conventions.
-fn determine_output_path(input: &Path) -> PathBuf {
-    let file_name = input.file_name().unwrap_or_default().to_string_lossy();
-
-    // Standard mappings
-    if file_name == ".env.example" || file_name == ".env.template" {
-        return input.with_file_name(".env");
-    }
-
-    if file_name == "docker-compose.yml" || file_name == "compose.yml" {
-        return input.with_file_name("compose.override.yml");
-    }
-    if file_name == "docker-compose.yaml" || file_name == "compose.yaml" {
-        return input.with_file_name("compose.override.yaml");
-    }
-
-    // Pattern-based mappings
-    if let Some(base) = file_name.strip_suffix(".env.example") {
-        return input.with_file_name(format!("{}.env", base));
-    }
-    if let Some(base) = file_name.strip_suffix(".env.template") {
-        return input.with_file_name(format!("{}.env", base));
-    }
-    if let Some(base) = file_name.strip_suffix(".example.json") {
-        return input.with_file_name(format!("{}.json", base));
-    }
-    if let Some(base) = file_name.strip_suffix(".template.json") {
-        return input.with_file_name(format!("{}.json", base));
-    }
-    if let Some(base) = file_name.strip_suffix(".example.yml") {
-        return input.with_file_name(format!("{}.yml", base));
-    }
-    if let Some(base) = file_name.strip_suffix(".template.yml") {
-        return input.with_file_name(format!("{}.yml", base));
-    }
-    if let Some(base) = file_name.strip_suffix(".example.yaml") {
-        return input.with_file_name(format!("{}.yaml", base));
-    }
-    if let Some(base) = file_name.strip_suffix(".template.yaml") {
-        return input.with_file_name(format!("{}.yaml", base));
-    }
-    if let Some(base) = file_name.strip_suffix(".example.toml") {
-        return input.with_file_name(format!("{}.toml", base));
-    }
-    if let Some(base) = file_name.strip_suffix(".template.toml") {
-        return input.with_file_name(format!("{}.toml", base));
-    }
-
-    input.with_extension(format!(
-        "{}.out",
-        input.extension().unwrap_or_default().to_string_lossy()
-    ))
-}
-
-/// Discovers common configuration template files in the current directory.
-fn find_input_file() -> Option<PathBuf> {
-    let candidates = [
-        ".env.example",
-        "compose.yml",
-        "docker-compose.yml",
-        ".env.template",
-        "compose.yaml",
-        "docker-compose.yaml",
-    ];
-
-    // Priority 1: Exact matches for well-known defaults
-    for name in &candidates {
-        let path = PathBuf::from(name);
-        if path.exists() {
-            return Some(path);
-        }
-    }
-
-    // Priority 2: Pattern matches
-    if let Ok(entries) = std::fs::read_dir(".") {
-        let mut fallback = None;
-        for entry in entries.flatten() {
-            let name = entry.file_name();
-            let name_str = name.to_string_lossy();
-
-            if name_str.ends_with(".env.example")
-                || name_str.ends_with(".env.template")
-                || name_str.ends_with(".example.json")
-                || name_str.ends_with(".template.json")
-                || name_str.ends_with(".example.yml")
-                || name_str.ends_with(".template.yml")
-                || name_str.ends_with(".example.yaml")
-                || name_str.ends_with(".template.yaml")
-                || name_str.ends_with(".example.toml")
-                || name_str.ends_with(".template.toml")
-            {
-                // Prefer .env.* or compose.* if multiple matches
-                if name_str.contains(".env") || name_str.contains("compose") {
-                    return Some(entry.path());
-                }
-                if fallback.is_none() {
-                    fallback = Some(entry.path());
-                }
-            }
-        }
-        if let Some(path) = fallback {
-            return Some(path);
-        }
-    }
-
-    None
-}
-
 fn main() -> anyhow::Result<()> {
     let args = cli::parse();
 
@@ -152,7 +45,7 @@ fn main() -> anyhow::Result<()> {
             }
             path
         }
-        None => match find_input_file() {
+        None => match resolver::find_input_file() {
             Some(path) => {
                 info!("Discovered template: {}", path.display());
                 path
@@ -172,44 +65,23 @@ fn main() -> anyhow::Result<()> {
     let handler = get_handler(format_type);
 
     // Smart Comparison Logic
-    let input_name = input_path.file_name().unwrap_or_default().to_string_lossy();
-    let is_template_input = input_name.contains(".example") || input_name.contains(".template") || input_name == "compose.yml" || input_name == "docker-compose.yml";
-    
-    let mut template_path = None;
-    let mut active_path = None;
-
-    if is_template_input {
-        template_path = Some(input_path.clone());
-        let expected_active = determine_output_path(&input_path);
-        if expected_active.exists() {
-            active_path = Some(expected_active);
-        }
-    } else {
-        // Input is likely an active config (e.g., .env)
-        active_path = Some(input_path.clone());
-        // Try to find a template
-        let possible_templates = [
-            format!("{}.example", input_name),
-            format!("{}.template", input_name),
-        ];
-        for t in possible_templates {
-            let p = input_path.with_file_name(t);
-            if p.exists() {
-                template_path = Some(p);
-                break;
-            }
-        }
-    }
+    let (active_path, template_path) = resolver::resolve_paths(&input_path);
 
     let output_path = args
         .output
-        .unwrap_or_else(|| active_path.clone().unwrap_or_else(|| determine_output_path(&input_path)));
+        .unwrap_or_else(|| active_path.clone().unwrap_or_else(|| resolver::determine_output_path(&input_path)));
 
     info!("Output: {}", output_path.display());
 
-    // 1. Load active config if it exists
+    // 1. Load active config if it exists.
+    //
+    // A parse failure must be fatal: opening an editor over a file we could not
+    // read would present an empty document, and the next `:w` would truncate a
+    // perfectly good config.
     let mut vars = if let Some(active) = &active_path {
-        handler.parse(active).unwrap_or_default()
+        handler
+            .parse(active)
+            .with_context(|| format!("could not parse {}", active.display()))?
     } else {
         Vec::new()
     };
@@ -217,60 +89,57 @@ fn main() -> anyhow::Result<()> {
     // 2. Load template config and merge
     if let Some(template) = &template_path {
         info!("Comparing with template: {}", template.display());
-        let template_vars = handler.parse(template).unwrap_or_default();
         if vars.is_empty() {
-             vars = template_vars;
-             // If we only have template, everything is missing from active initially
-             for v in vars.iter_mut() {
-                 v.status = crate::format::ItemStatus::MissingFromActive;
-                 v.value = None;
-             }
+            vars = handler
+                .parse(template)
+                .with_context(|| format!("could not parse template {}", template.display()))?;
+            // Only the template exists, so nothing has been accepted into the
+            // active config yet. Keep each template value as the default on offer.
+            for v in vars.iter_mut() {
+                if !v.is_group {
+                    v.status = crate::format::ItemStatus::MissingFromActive;
+                    v.template_value = v.value.take();
+                    v.default_value = v.template_value.clone();
+                }
+            }
         } else {
-             // Merge template into active
-             handler.merge(template, &mut vars).unwrap_or_default();
+            handler
+                .merge(template, &mut vars)
+                .with_context(|| format!("could not parse template {}", template.display()))?;
         }
     } else if vars.is_empty() {
         // Fallback if no template and active is empty
-         vars = handler.parse(&input_path).map_err(|e| {
-            error!("Failed to parse input file: {}", e);
-            MouldError::Format(format!("Failed to parse {}: {}", input_path.display(), e))
-        })?;
+        vars = handler
+            .parse(&input_path)
+            .with_context(|| format!("could not parse {}", input_path.display()))?;
     }
 
     if vars.is_empty() {
         warn!("No variables found.");
     }
 
-    let config = load_config();
-    let mut app = App::new(vars);
+    let (config, config_error) = load_config();
+    let mut app = App::new(vars, output_path.clone(), template_path.clone());
+    if let Some(err) = config_error {
+        app.set_status(format!("config.toml ignored: {}", err));
+    }
 
     // Terminal lifecycle
     enable_raw_mode()
         .map_err(|e| MouldError::Terminal(format!("Failed to enable raw mode: {}", e)))?;
     let mut stdout = io::stdout();
-    execute!(stdout, EnterAlternateScreen, EnableMouseCapture)
+    execute!(stdout, EnterAlternateScreen)
         .map_err(|e| MouldError::Terminal(format!("Failed to enter alternate screen: {}", e)))?;
     let backend = CrosstermBackend::new(stdout);
     let mut terminal = Terminal::new(backend)
         .map_err(|e| MouldError::Terminal(format!("Failed to create terminal backend: {}", e)))?;
 
-    let mut runner = AppRunner::new(
-        &mut terminal,
-        &mut app,
-        &config,
-        &output_path,
-        handler.as_ref(),
-    );
+    let mut runner = AppRunner::new(&mut terminal, &mut app, &config, handler.as_ref());
     let res = runner.run();
 
     // Restoration
     disable_raw_mode().ok();
-    execute!(
-        terminal.backend_mut(),
-        LeaveAlternateScreen,
-        DisableMouseCapture
-    )
-    .ok();
+    execute!(terminal.backend_mut(), LeaveAlternateScreen).ok();
     terminal.show_cursor().ok();
 
     match res {
@@ -283,4 +152,4 @@ fn main() -> anyhow::Result<()> {
             Err(e.into())
         }
     }
-}
+}

src/resolver.rs

@@ -0,0 +1,294 @@
+//! Automatically resolves relationships between template and active configuration files.
+//!
+//! The resolver allows `mould` to be run without explicit output arguments
+//! by intelligently guessing the counterpart of a given input file based
+//! on common naming conventions.
+
+use std::path::{Path, PathBuf};
+
+pub struct Rule {
+    pub template_suffix: &'static str,
+    pub active_suffix: &'static str,
+    pub is_exact_match: bool,
+}
+
+pub const RULES: &[Rule] = &[
+    // Exact matches
+    Rule {
+        template_suffix: "compose.yml",
+        active_suffix: "compose.override.yml",
+        is_exact_match: true,
+    },
+    Rule {
+        template_suffix: "compose.yaml",
+        active_suffix: "compose.override.yaml",
+        is_exact_match: true,
+    },
+    Rule {
+        template_suffix: "docker-compose.yml",
+        active_suffix: "docker-compose.override.yml",
+        is_exact_match: true,
+    },
+    Rule {
+        template_suffix: "docker-compose.yaml",
+        active_suffix: "docker-compose.override.yaml",
+        is_exact_match: true,
+    },
+    // Pattern matches
+    Rule {
+        template_suffix: ".env.example",
+        active_suffix: ".env",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".env.template",
+        active_suffix: ".env",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".example.json",
+        active_suffix: ".json",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".template.json",
+        active_suffix: ".json",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".example.yml",
+        active_suffix: ".yml",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".template.yml",
+        active_suffix: ".yml",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".example.yaml",
+        active_suffix: ".yaml",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".template.yaml",
+        active_suffix: ".yaml",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".example.toml",
+        active_suffix: ".toml",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".template.toml",
+        active_suffix: ".toml",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".example.xml",
+        active_suffix: ".xml",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".template.xml",
+        active_suffix: ".xml",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".example.ini",
+        active_suffix: ".ini",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".template.ini",
+        active_suffix: ".ini",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".example.properties",
+        active_suffix: ".properties",
+        is_exact_match: false,
+    },
+    Rule {
+        template_suffix: ".template.properties",
+        active_suffix: ".properties",
+        is_exact_match: false,
+    },
+];
+
+pub const DEFAULT_CANDIDATES: &[&str] = &[
+    ".env.example",
+    "compose.yml",
+    "docker-compose.yml",
+    ".env.template",
+    "compose.yaml",
+    "docker-compose.yaml",
+];
+
+/// Helper to automatically determine the output file path based on common naming conventions.
+pub fn determine_output_path(input: &Path) -> PathBuf {
+    let file_name = input.file_name().unwrap_or_default().to_string_lossy();
+
+    for rule in RULES {
+        if rule.is_exact_match {
+            if file_name == rule.template_suffix {
+                return input.with_file_name(rule.active_suffix);
+            }
+        } else if file_name == rule.template_suffix {
+            return input.with_file_name(rule.active_suffix);
+        } else if let Some(base) = file_name.strip_suffix(rule.template_suffix) {
+            return input.with_file_name(format!("{}{}", base, rule.active_suffix));
+        }
+    }
+
+    input.with_extension(format!(
+        "{}.out",
+        input.extension().unwrap_or_default().to_string_lossy()
+    ))
+}
+
+/// Discovers common configuration template files in the current directory.
+pub fn find_input_file() -> Option<PathBuf> {
+    // Priority 1: Exact matches for well-known defaults
+    for &name in DEFAULT_CANDIDATES {
+        let path = PathBuf::from(name);
+        if path.exists() {
+            return Some(path);
+        }
+    }
+
+    // Priority 2: Pattern matches
+    if let Ok(entries) = std::fs::read_dir(".") {
+        let mut fallback = None;
+        for entry in entries.flatten() {
+            let name = entry.file_name();
+            let name_str = name.to_string_lossy();
+
+            for rule in RULES {
+                if !rule.is_exact_match && name_str.ends_with(rule.template_suffix) {
+                    if name_str.contains(".env") || name_str.contains("compose") {
+                        return Some(entry.path());
+                    }
+                    if fallback.is_none() {
+                        fallback = Some(entry.path());
+                    }
+                    break;
+                }
+            }
+        }
+        if let Some(path) = fallback {
+            return Some(path);
+        }
+    }
+
+    None
+}
+
+/// Resolves the active and template paths given an input path.
+/// Returns `(active_path, template_path)`.
+pub fn resolve_paths(input: &Path) -> (Option<PathBuf>, Option<PathBuf>) {
+    let file_name = input.file_name().unwrap_or_default().to_string_lossy();
+
+    // Check if the input matches any known template pattern
+    let mut is_template = false;
+    for rule in RULES {
+        if rule.is_exact_match {
+            if file_name == rule.template_suffix {
+                is_template = true;
+                break;
+            }
+        } else if file_name.ends_with(rule.template_suffix) {
+            is_template = true;
+            break;
+        }
+    }
+
+    // Fallback template detection
+    if !is_template && (file_name.contains(".example") || file_name.contains(".template")) {
+        is_template = true;
+    }
+
+    if is_template {
+        let expected_active = determine_output_path(input);
+        let active = if expected_active.exists() {
+            Some(expected_active)
+        } else {
+            None
+        };
+        (active, Some(input.to_path_buf()))
+    } else {
+        // Input is treated as the active config
+        let active = Some(input.to_path_buf());
+        let mut template = None;
+
+        // Try to reverse match rules to find a template
+        for rule in RULES {
+            if rule.is_exact_match {
+                if file_name == rule.active_suffix {
+                    let t = input.with_file_name(rule.template_suffix);
+                    if t.exists() {
+                        template = Some(t);
+                        break;
+                    }
+                }
+            } else if file_name.ends_with(rule.active_suffix) {
+                if file_name == rule.active_suffix {
+                    let t = input.with_file_name(rule.template_suffix);
+                    if t.exists() {
+                        template = Some(t);
+                        break;
+                    }
+                } else if let Some(base) = file_name.strip_suffix(rule.active_suffix) {
+                    let t = input.with_file_name(format!("{}{}", base, rule.template_suffix));
+                    if t.exists() {
+                        template = Some(t);
+                        break;
+                    }
+                }
+            }
+        }
+
+        // Fallback reverse detection
+        if template.is_none() {
+            let possible_templates = [
+                format!("{}.example", file_name),
+                format!("{}.template", file_name),
+            ];
+            for t in possible_templates {
+                let p = input.with_file_name(t);
+                if p.exists() {
+                    template = Some(p);
+                    break;
+                }
+            }
+        }
+
+        (active, template)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_determine_output_path() {
+        assert_eq!(
+            determine_output_path(Path::new(".env.example")),
+            PathBuf::from(".env")
+        );
+        assert_eq!(
+            determine_output_path(Path::new("compose.yml")),
+            PathBuf::from("compose.override.yml")
+        );
+        assert_eq!(
+            determine_output_path(Path::new("config.template.json")),
+            PathBuf::from("config.json")
+        );
+        assert_eq!(
+            determine_output_path(Path::new("config.example")),
+            PathBuf::from("config.example.out")
+        );
+    }
+}

src/runner.rs

@@ -1,28 +1,37 @@
+//! Orchestrates the main application execution, terminal events, and TUI rendering.
+//!
+//! The `AppRunner` owns the event loop, turns raw key events into the
+//! [`Action`]s declared in the user's config, and runs the `:` command line.
+
 use crate::app::{App, InsertVariant, Mode};
-use crate::config::Config;
+use crate::config::{Action, Config};
 use crate::format::FormatHandler;
-use crossterm::event::{self, Event, KeyCode, KeyEvent};
+use crossterm::event::{self, Event, KeyCode, KeyEvent, KeyEventKind, KeyModifiers};
 use ratatui::Terminal;
 use ratatui::backend::Backend;
 use std::io;
-use std::path::Path;
+use std::path::PathBuf;
+use std::time::Duration;
 use tui_input::backend::crossterm::EventHandler;
 
+/// How often the loop wakes up with no input, so timed UI state can settle.
+const TICK: Duration = Duration::from_millis(250);
+
 /// Manages the main application execution loop, event handling, and terminal interaction.
 pub struct AppRunner<'a, B: Backend> {
     /// Reference to the terminal instance.
     terminal: &'a mut Terminal<B>,
     /// Mutable reference to the application state.
     app: &'a mut App,
-    /// Loaded user configuration.
+    /// Loaded user configuration (keybinds, theme).
     config: &'a Config,
-    /// Path where the final configuration will be saved.
-    output_path: &'a Path,
     /// Handler for the specific file format (env, json, yaml, toml).
     handler: &'a dyn FormatHandler,
+    /// Every key sequence and the action it triggers, resolved from the config once.
+    bindings: Vec<(String, Action)>,
     /// Buffer for storing active command entry (e.g., ":w").
     command_buffer: String,
-    /// Buffer for storing sequence of key presses (e.g., "gg").
+    /// Buffer for storing multi-key sequence of presses (e.g., "gg").
     key_sequence: String,
 }
 
@@ -30,47 +39,78 @@ impl<'a, B: Backend> AppRunner<'a, B>
 where
     io::Error: From<B::Error>,
 {
-    /// Creates a new runner instance.
+    /// Creates a new runner instance with all required dependencies.
     pub fn new(
         terminal: &'a mut Terminal<B>,
         app: &'a mut App,
         config: &'a Config,
-        output_path: &'a Path,
         handler: &'a dyn FormatHandler,
     ) -> Self {
         Self {
             terminal,
             app,
             config,
-            output_path,
             handler,
+            bindings: config.keybinds.resolved(),
             command_buffer: String::new(),
             key_sequence: String::new(),
         }
     }
 
     /// Starts the main application loop.
+    /// 
+    /// This loop continues until `self.app.running` is set to false. It polls
+    /// rather than blocking so that a resize or an expiring status message
+    /// repaints without waiting for the next keystroke.
     pub fn run(&mut self) -> io::Result<()> {
         while self.app.running {
             self.terminal
                 .draw(|f| crate::ui::draw(f, self.app, self.config))?;
 
-            if let Event::Key(key) = event::read()? {
-                self.handle_key_event(key)?;
+            if !event::poll(TICK)? {
+                continue;
+            }
+
+            match event::read()? {
+                // Key *releases* are delivered on some terminals; acting on both
+                // edges would fire every binding twice.
+                Event::Key(key) if key.kind != KeyEventKind::Release => {
+                    self.handle_key_event(key)?;
+                }
+                _ => {}
             }
         }
         Ok(())
     }
 
     /// Primary dispatcher for all keyboard events.
+    ///
+    /// It delegates handling to specialized methods based on the 
+    /// current application mode.
     fn handle_key_event(&mut self, key: KeyEvent) -> io::Result<()> {
         match self.app.mode {
             Mode::Normal => self.handle_normal_mode(key),
-            Mode::Insert => self.handle_insert_mode(key),
+            Mode::Insert | Mode::InsertKey => self.handle_insert_mode(key),
             Mode::Search => self.handle_search_mode(key),
+            Mode::Help => self.handle_help_mode(key),
         }
     }
 
+    /// The help overlay scrolls with the usual movement keys; anything else closes it.
+    fn handle_help_mode(&mut self, key: KeyEvent) -> io::Result<()> {
+        let token = Self::key_token(key);
+        let is = |action| token.as_deref() == Some(self.config.keybinds.key_for(action)).as_deref();
+
+        if is(Action::Down) || key.code == KeyCode::Down {
+            self.app.scroll_help(true);
+        } else if is(Action::Up) || key.code == KeyCode::Up {
+            self.app.scroll_help(false);
+        } else {
+            self.app.toggle_help();
+        }
+        Ok(())
+    }
+
     /// Handles keys in Normal mode, separating navigation from command entry.
     fn handle_normal_mode(&mut self, key: KeyEvent) -> io::Result<()> {
         if !self.command_buffer.is_empty() {
@@ -86,135 +126,156 @@ where
             KeyCode::Enter => {
                 let cmd = self.command_buffer.clone();
                 self.command_buffer.clear();
-                self.execute_command(&cmd)
+                self.app.prompt = None;
+                self.execute_command(cmd.trim())
             }
             KeyCode::Esc => {
                 self.command_buffer.clear();
-                self.app.status_message = None;
+                self.app.prompt = None;
+                self.app.clear_status();
                 Ok(())
             }
             KeyCode::Backspace => {
                 self.command_buffer.pop();
-                self.sync_command_status();
+                self.sync_command_prompt();
                 Ok(())
             }
             KeyCode::Char(c) => {
                 self.command_buffer.push(c);
-                self.sync_command_status();
+                self.sync_command_prompt();
                 Ok(())
             }
             _ => Ok(()),
         }
     }
 
-    /// Handles primary navigation (j/k) and transitions to insert or command modes.
-    fn handle_navigation_mode(&mut self, key: KeyEvent) -> io::Result<()> {
-        if let KeyCode::Char(c) = key.code {
-            self.key_sequence.push(c);
+    /// Renders a key event as the string used in the keybinding config.
+    ///
+    /// Named keys are spelled in angle brackets (`<Space>`) so that they can
+    /// never be confused with a sequence of ordinary characters.
+    fn key_token(key: KeyEvent) -> Option<String> {
+        let base = match key.code {
+            KeyCode::Char(' ') => "<Space>".to_string(),
+            KeyCode::Char(c) => c.to_string(),
+            KeyCode::Tab => "<Tab>".to_string(),
+            KeyCode::BackTab => "<S-Tab>".to_string(),
+            _ => return None,
+        };
 
-            // Collect all configured keybinds
-            let binds = [
-                (&self.config.keybinds.down, "down"),
-                (&self.config.keybinds.up, "up"),
-                (&self.config.keybinds.edit, "edit"),
-                (&self.config.keybinds.edit_append, "edit_append"),
-                (&self.config.keybinds.edit_substitute, "edit_substitute"),
-                (&self.config.keybinds.search, "search"),
-                (&self.config.keybinds.next_match, "next_match"),
-                (&self.config.keybinds.previous_match, "previous_match"),
-                (&self.config.keybinds.jump_top, "jump_top"),
-                (&self.config.keybinds.jump_bottom, "jump_bottom"),
-                (&self.config.keybinds.append_item, "append_item"),
-                (&self.config.keybinds.prepend_item, "prepend_item"),
-                (&self.config.keybinds.delete_item, "delete_item"),
-                (&self.config.keybinds.undo, "undo"),
-                (&"a".to_string(), "add_missing"),
-                (&":".to_string(), "command"),
-                (&"q".to_string(), "quit"),
-            ];
-
-            let mut exact_match = None;
-            let mut prefix_match = false;
-
-            for (bind, action) in binds.iter() {
-                if bind == &&self.key_sequence {
-                    exact_match = Some(*action);
-                    break;
-                } else if bind.starts_with(&self.key_sequence) {
-                    prefix_match = true;
-                }
-            }
-
-            if let Some(action) = exact_match {
-                self.key_sequence.clear();
-                match action {
-                    "down" => self.app.next(),
-                    "up" => self.app.previous(),
-                    "edit" => self.app.enter_insert(InsertVariant::Start),
-                    "edit_append" => self.app.enter_insert(InsertVariant::End),
-                    "edit_substitute" => self.app.enter_insert(InsertVariant::Substitute),
-                    "search" => {
-                        self.app.mode = Mode::Search;
-                        self.app.search_query.clear();
-                        self.app.status_message = Some(format!("{} ", self.config.keybinds.search));
-                    }
-                    "next_match" => self.app.jump_next_match(),
-                    "previous_match" => self.app.jump_previous_match(),
-                    "jump_top" => self.app.jump_top(),
-                    "jump_bottom" => self.app.jump_bottom(),
-                    "append_item" => self.app.add_array_item(true),
-                    "prepend_item" => self.app.add_array_item(false),
-                    "delete_item" => self.app.delete_selected(),
-                    "undo" => self.app.undo(),
-                    "add_missing" => {
-                        self.app.save_undo_state();
-                        self.add_missing_item();
-                    }
-                    "command" => {
-                        self.command_buffer.push(':');
-                        self.sync_command_status();
-                    }
-                    "quit" => self.app.running = false,
-                    _ => {}
-                }
-            } else if !prefix_match {
-                // Not an exact match and not a prefix for any bind, clear and restart seq
-                self.key_sequence.clear();
-                self.key_sequence.push(c);
-            }
+        if key.modifiers.contains(KeyModifiers::ALT) {
+            Some(format!("alt+{}", base))
         } else {
-            // Non-character keys reset the sequence buffer
+            Some(base)
+        }
+    }
+
+    /// Handles primary navigation and transitions to insert or command modes.
+    ///
+    /// This method manages multi-key sequences (like `gg`) and immediate 
+    /// actions (like `i`). It correctly re-evaluates sequences to prevent 
+    /// "one-key-behind" responsiveness bugs.
+    fn handle_navigation_mode(&mut self, key: KeyEvent) -> io::Result<()> {
+        let Some(token) = Self::key_token(key) else {
+            // Reset the sequence buffer if a non-character key is pressed.
             self.key_sequence.clear();
             match key.code {
                 KeyCode::Down => self.app.next(),
                 KeyCode::Up => self.app.previous(),
-                KeyCode::Enter => self.save_file()?,
-                KeyCode::Esc => self.app.status_message = None,
+                KeyCode::Enter => {
+                    self.save_file(None);
+                }
+                KeyCode::Esc => self.app.clear_status(),
                 _ => {}
             }
+            return Ok(());
+        };
+
+        self.key_sequence.push_str(&token);
+
+        let mut action = self.find_binding();
+        if action.is_none() {
+            if self.is_prefix_binding() {
+                // A partial multi-key bind (like the first 'g' of 'gg'): wait.
+                return Ok(());
+            }
+            // Not a match and not a prefix: restart from the current key.
+            self.key_sequence = token;
+            action = self.find_binding();
+            if action.is_none() && !self.is_prefix_binding() {
+                self.key_sequence.clear();
+            }
         }
+
+        let Some(action) = action else { return Ok(()) };
+        self.key_sequence.clear();
+        self.dispatch(action);
         Ok(())
     }
 
-    /// Adds a missing item from the template to the active configuration.
-    fn add_missing_item(&mut self) {
-        if let Some(var) = self.app.vars.get_mut(self.app.selected) {
-            if var.status == crate::format::ItemStatus::MissingFromActive {
-                var.status = crate::format::ItemStatus::Present;
-                if !var.is_group {
-                    var.value = var.template_value.clone();
-                }
-                self.app.sync_input_with_selected();
+    /// Applies a resolved action to the application state.
+    fn dispatch(&mut self, action: Action) {
+        match action {
+            Action::Down => self.app.next(),
+            Action::Up => self.app.previous(),
+            Action::JumpTop => self.app.jump_top(),
+            Action::JumpBottom => self.app.jump_bottom(),
+            Action::Edit => self.app.enter_insert(InsertVariant::Start),
+            Action::EditAppend => self.app.enter_insert(InsertVariant::End),
+            Action::EditSubstitute => self.app.enter_insert(InsertVariant::Substitute),
+            Action::Rename => self.app.enter_insert_key(),
+            Action::Search => {
+                self.app.begin_search();
+                self.sync_search_prompt();
             }
+            Action::NextMatch => self.app.jump_next_match(),
+            Action::PreviousMatch => self.app.jump_previous_match(),
+            Action::AppendItem => self.app.add_item(true, false, false),
+            Action::PrependItem => self.app.add_item(false, false, false),
+            Action::AppendGroup => self.app.add_item(true, true, true),
+            Action::PrependGroup => self.app.add_item(false, true, true),
+            Action::DeleteItem => self.app.delete_selected(),
+            Action::ToggleGroup => {
+                self.app.toggle_group_selected();
+                self.app.save_undo_state();
+            }
+            Action::AddMissing => self.app.accept_selected(),
+            Action::AcceptAll => self.app.accept_all_missing(),
+            Action::ToggleFold => self.app.toggle_fold(),
+            Action::FoldAll => self.app.set_all_folds(true),
+            Action::UnfoldAll => self.app.set_all_folds(false),
+            Action::Undo => self.app.undo(),
+            Action::Redo => self.app.redo(),
+            Action::UndoBranchPrev => self.app.cycle_undo_branch(false),
+            Action::UndoBranchNext => self.app.cycle_undo_branch(true),
+            Action::Help => self.app.toggle_help(),
+            Action::Command => {
+                self.command_buffer.push(':');
+                self.sync_command_prompt();
+            }
+            Action::Quit => self.request_quit(false),
         }
     }
 
+    /// Looks up the current `key_sequence` in the configured keybindings.
+    fn find_binding(&self) -> Option<Action> {
+        self.bindings
+            .iter()
+            .find(|(bind, _)| *bind == self.key_sequence)
+            .map(|(_, action)| *action)
+    }
+
+    /// Returns true if the current `key_sequence` is a partial prefix of any configured bind.
+    fn is_prefix_binding(&self) -> bool {
+        self.bindings
+            .iter()
+            .any(|(bind, _)| bind.starts_with(&self.key_sequence) && bind != &self.key_sequence)
+    }
+
     /// Delegates key events to the `tui_input` handler during active editing.
     fn handle_insert_mode(&mut self, key: KeyEvent) -> io::Result<()> {
         match key.code {
-            KeyCode::Esc | KeyCode::Enter => {
-                self.app.enter_normal();
-            }
+            KeyCode::Esc => self.app.cancel_insert(),
+            KeyCode::Enter => self.app.enter_normal(),
             _ => {
                 self.app.input.handle_event(&Event::Key(key));
             }
@@ -222,21 +283,21 @@ where
         Ok(())
     }
 
-    /// Handles search mode key events.
+    /// Handles search mode key events and live-updates the match position.
     fn handle_search_mode(&mut self, key: KeyEvent) -> io::Result<()> {
         match key.code {
-            KeyCode::Enter | KeyCode::Esc => {
-                self.app.mode = Mode::Normal;
-                self.app.status_message = None;
-            }
+            // Enter keeps the query so `n`/`N` continue to work; Esc abandons it
+            // and puts the cursor back where the search started.
+            KeyCode::Enter => self.app.end_search(true),
+            KeyCode::Esc => self.app.end_search(false),
             KeyCode::Backspace => {
                 self.app.search_query.pop();
-                self.app.status_message = Some(format!("{}{}", self.config.keybinds.search, self.app.search_query));
+                self.sync_search_prompt();
                 self.app.jump_next_match();
             }
             KeyCode::Char(c) => {
                 self.app.search_query.push(c);
-                self.app.status_message = Some(format!("{}{}", self.config.keybinds.search, self.app.search_query));
+                self.sync_search_prompt();
                 self.app.jump_next_match();
             }
             _ => {}
@@ -244,39 +305,85 @@ where
         Ok(())
     }
 
-    /// Logic to map command strings (like ":w") to internal application actions.
+    /// Maps command strings (like `:w`) to internal application actions.
     fn execute_command(&mut self, cmd: &str) -> io::Result<()> {
-        if cmd == self.config.keybinds.save {
-            self.save_file()
-        } else if cmd == self.config.keybinds.quit {
-            self.app.running = false;
-            Ok(())
-        } else if cmd == ":wq" {
-            self.save_file()?;
-            self.app.running = false;
-            Ok(())
-        } else {
-            self.app.status_message = Some(format!("Unknown command: {}", cmd));
-            Ok(())
-        }
-    }
+        let kb = &self.config.keybinds;
+        let save = kb.save.clone();
+        let quit = kb.quit.clone();
 
-    /// Attempts to write the current app state to the specified output file.
-    fn save_file(&mut self) -> io::Result<()> {
-        if self.handler.write(self.output_path, &self.app.vars).is_ok() {
-            self.app.status_message = Some(format!("Saved to {}", self.output_path.display()));
-        } else {
-            self.app.status_message = Some("Error saving file".to_string());
+        // `:w some/other/path` retargets the write without changing the session.
+        if let Some(rest) = cmd.strip_prefix(&format!("{} ", save)) {
+            let target = PathBuf::from(rest.trim());
+            self.save_file(Some(target));
+            return Ok(());
+        }
+
+        match cmd {
+            c if c == save => {
+                self.save_file(None);
+            }
+            c if c == quit => self.request_quit(false),
+            ":q!" => self.request_quit(true),
+            ":wq" | ":x" => {
+                if self.save_file(None) {
+                    self.app.running = false;
+                }
+            }
+            ":help" | ":h" => self.app.toggle_help(),
+            other => self.app.set_status(format!("Unknown command: {}", other)),
         }
         Ok(())
     }
 
-    /// Synchronizes the status bar display with the active command buffer.
-    fn sync_command_status(&mut self) {
-        if self.command_buffer.is_empty() {
-            self.app.status_message = None;
-        } else {
-            self.app.status_message = Some(self.command_buffer.clone());
+    /// Quits, refusing to discard unsaved work unless `force` is set.
+    fn request_quit(&mut self, force: bool) {
+        if self.app.is_dirty && !force {
+            self.app.set_status(format!(
+                "Unsaved changes — {} to save, :q! to discard",
+                self.config.keybinds.save
+            ));
+            return;
+        }
+        self.app.running = false;
+    }
+
+    /// Writes the current state to disk. Returns true on success.
+    fn save_file(&mut self, target: Option<PathBuf>) -> bool {
+        let path = target.unwrap_or_else(|| self.app.output_path.clone());
+        match self.handler.write(&path, &self.app.vars) {
+            Ok(()) => {
+                self.app.output_path = path.clone();
+                self.app.is_dirty = false;
+                for var in self.app.vars.iter_mut() {
+                    var.dirty = false;
+                }
+                self.app.set_status(format!("Saved to {}", path.display()));
+                true
+            }
+            Err(e) => {
+                // The reason matters: a read-only path and an unrepresentable
+                // document fail very differently.
+                self.app.set_status(format!("Could not save {}: {}", path.display(), e));
+                false
+            }
         }
     }
+
+    /// Synchronizes the prompt line with the active command buffer.
+    fn sync_command_prompt(&mut self) {
+        self.app.prompt = if self.command_buffer.is_empty() {
+            None
+        } else {
+            Some(self.command_buffer.clone())
+        };
+    }
+
+    /// Synchronizes the prompt line with the active search query.
+    fn sync_search_prompt(&mut self) {
+        self.app.prompt = Some(format!(
+            "{}{}",
+            self.config.keybinds.key_for(Action::Search),
+            self.app.search_query
+        ));
+    }
 }

src/ui.rs

@@ -1,11 +1,18 @@
+//! Renders the hierarchical Terminal User Interface (TUI).
+//!
+//! The screen is four things stacked: a header naming the files in play, the
+//! item tree, a detail/edit pane for the selection, and a status bar. Nothing
+//! here mutates state beyond the widget list itself.
+
 use crate::app::{App, Mode};
-use crate::config::Config;
+use crate::config::{Action, Config, ThemeConfig};
+use crate::format::{ItemStatus, ValueType};
 use ratatui::{
     Frame,
-    layout::{Constraint, Direction, Layout},
+    layout::{Alignment, Constraint, Direction, Layout, Rect},
     style::{Modifier, Style},
     text::{Line, Span},
-    widgets::{Block, BorderType, Borders, List, ListItem, ListState, Paragraph},
+    widgets::{Block, BorderType, Borders, Clear, List, ListItem, ListState, Paragraph},
 };
 
 /// Renders the main application interface using ratatui.
@@ -13,7 +20,7 @@ pub fn draw(f: &mut Frame, app: &mut App, config: &Config) {
     let theme = &config.theme;
     let size = f.area();
 
-    // Render the main background (optional based on transparency config).
+    // Render the main background.
     if !theme.transparent {
         f.render_widget(
             Block::default().style(Style::default().bg(theme.bg_normal())),
@@ -21,7 +28,7 @@ pub fn draw(f: &mut Frame, app: &mut App, config: &Config) {
         );
     }
 
-    // Horizontal layout with 1-character side margins.
+    // Define outer margins.
     let outer_layout = Layout::default()
         .direction(Direction::Horizontal)
         .constraints([
@@ -31,54 +38,54 @@ pub fn draw(f: &mut Frame, app: &mut App, config: &Config) {
         ])
         .split(size);
 
-    // Vertical layout for the main UI components.
+    // Split the center area vertically.
     let chunks = Layout::default()
         .direction(Direction::Vertical)
         .constraints([
             Constraint::Length(1), // Top margin
-            Constraint::Min(3),    // Main list
-            Constraint::Length(3), // Focused input field
+            Constraint::Min(3),    // Main tree list
+            Constraint::Length(3), // Detail / edit pane
             Constraint::Length(1), // Spacer
             Constraint::Length(1), // Status bar
         ])
         .split(outer_layout[1]);
 
-    // Build the interactive list of configuration variables.
-    let matching_indices = app.matching_indices();
-    let items: Vec<ListItem> = app
-        .vars
+    draw_tree(f, chunks[1], app, config);
+    draw_detail(f, chunks[2], app, theme);
+    draw_status(f, chunks[4], app, config);
+
+    if app.mode == Mode::Help {
+        draw_help(f, size, config, app.help_scroll);
+    }
+}
+
+/// Renders the item tree, skipping anything inside a folded group.
+fn draw_tree(f: &mut Frame, area: Rect, app: &App, config: &Config) {
+    let theme = &config.theme;
+    let matching = app.matching_indices();
+    let visible = app.visible_indices();
+
+    let items: Vec<ListItem> = visible
         .iter()
-        .enumerate()
-        .map(|(i, var)| {
+        .map(|&i| {
+            let var = &app.vars[i];
             let is_selected = i == app.selected;
-            let is_match = matching_indices.contains(&i);
+            let is_match = matching.contains(&i);
+            let is_missing = var.status == ItemStatus::MissingFromActive;
 
-            // Indentation based on depth
             let indent = "  ".repeat(var.depth);
-            let prefix = if var.is_group { "+ " } else { "  " };
-
-            // Determine colors based on depth
-            let depth_color = if is_selected {
-                theme.bg_normal()
+            // A group that can be folded says so; leaves are just indented.
+            let prefix = if var.is_group {
+                if var.collapsed { "▸ " } else { "▾ " }
             } else {
-                match var.depth % 4 {
-                    0 => theme.tree_depth_1(),
-                    1 => theme.tree_depth_2(),
-                    2 => theme.tree_depth_3(),
-                    3 => theme.tree_depth_4(),
-                    _ => theme.fg_normal(),
-                }
+                "  "
             };
 
-            // Determine colors based on status and selection
-            let text_color = if is_selected {
-                theme.fg_highlight()
-            } else {
-                match var.status {
-                    crate::format::ItemStatus::MissingFromActive if !var.is_group => theme.fg_dimmed(),
-                    crate::format::ItemStatus::Modified => theme.fg_modified(),
-                    _ => theme.fg_normal(),
-                }
+            let depth_color = match var.depth % 4 {
+                0 => theme.tree_depth_1(),
+                1 => theme.tree_depth_2(),
+                2 => theme.tree_depth_3(),
+                _ => theme.tree_depth_4(),
             };
 
             let key_style = if is_selected {
@@ -89,7 +96,7 @@ pub fn draw(f: &mut Frame, app: &mut App, config: &Config) {
                 Style::default()
                     .fg(theme.bg_search())
                     .add_modifier(Modifier::UNDERLINED)
-            } else if var.status == crate::format::ItemStatus::MissingFromActive && !var.is_group {
+            } else if is_missing {
                 Style::default()
                     .fg(theme.fg_dimmed())
                     .add_modifier(Modifier::DIM)
@@ -98,71 +105,90 @@ pub fn draw(f: &mut Frame, app: &mut App, config: &Config) {
             };
 
             let mut key_spans = vec![
-                Span::raw(indent),
+                Span::raw(indent.clone()),
                 Span::styled(prefix, Style::default().fg(theme.border_normal())),
                 Span::styled(&var.key, key_style),
             ];
 
-            // Add status indicator if not present (only for leaf variables)
-            if !var.is_group {
-                match var.status {
-                    crate::format::ItemStatus::MissingFromActive => {
-                        let missing_style = if is_selected {
-                            Style::default().fg(theme.fg_highlight()).add_modifier(Modifier::BOLD)
-                        } else {
-                            Style::default().fg(theme.fg_warning()).add_modifier(Modifier::BOLD)
-                        };
-                        key_spans.push(Span::styled(" (missing)", missing_style));
-                    }
-                    crate::format::ItemStatus::Modified => {
-                        if !is_selected {
-                            key_spans.push(Span::styled(" (*)", Style::default().fg(theme.fg_modified())));
-                        }
-                    }
-                    _ => {}
+            let tag_style = |color| {
+                if is_selected {
+                    Style::default()
+                        .fg(theme.fg_highlight())
+                        .add_modifier(Modifier::BOLD)
+                } else {
+                    Style::default().fg(color).add_modifier(Modifier::BOLD)
                 }
+            };
+
+            if is_missing {
+                let label = if var.is_group { " (missing group)" } else { " (missing)" };
+                key_spans.push(Span::styled(label, tag_style(theme.fg_warning())));
+            }
+            // "changed" means changed by you, in this session — not merely
+            // different from the template, which is the normal case.
+            if var.dirty {
+                key_spans.push(Span::styled(" (*)", tag_style(theme.fg_modified())));
+            }
+            if var.is_group && var.collapsed {
+                let hidden = app.vars.iter().filter(|v| v.is_descendant_of(&var.path)).count();
+                key_spans.push(Span::styled(
+                    format!("  {} hidden", hidden),
+                    Style::default().fg(theme.fg_dimmed()),
+                ));
             }
 
             let item_style = if is_selected {
                 Style::default().bg(theme.bg_highlight())
             } else {
-                Style::default().fg(text_color)
+                Style::default().fg(theme.fg_normal())
             };
 
             if var.is_group {
-                ListItem::new(Line::from(key_spans)).style(item_style)
-            } else {
-                // Show live input text for the selected item if in Insert mode.
-                let val = if is_selected && matches!(app.mode, Mode::Insert) {
-                    app.input.value()
-                } else {
-                    var.value.as_deref().unwrap_or("")
-                };
-
-                let value_style = if is_selected {
-                    Style::default().fg(theme.fg_highlight())
-                } else {
-                    Style::default().fg(theme.fg_normal())
-                };
-
-                let mut val_spans = vec![
-                    Span::raw(format!("{}    └─ ", "  ".repeat(var.depth))),
-                    Span::styled(val, value_style),
-                ];
-
-                if let Some(t_val) = &var.template_value {
-                    if Some(t_val) != var.value.as_ref() {
-                        let t_style = if is_selected {
-                            Style::default().fg(theme.bg_normal()).add_modifier(Modifier::DIM)
-                        } else {
-                            Style::default().fg(theme.fg_dimmed()).add_modifier(Modifier::ITALIC)
-                        };
-                        val_spans.push(Span::styled(format!(" [Def: {}]", t_val), t_style));
-                    }
-                }
-
-                ListItem::new(vec![Line::from(key_spans), Line::from(val_spans)]).style(item_style)
+                return ListItem::new(Line::from(key_spans)).style(item_style);
             }
+
+            // Value line.
+            let live_edit = is_selected && app.mode == Mode::Insert;
+            let value_style = if is_selected {
+                Style::default().fg(theme.fg_highlight())
+            } else {
+                Style::default().fg(theme.fg_normal())
+            };
+
+            let mut val_spans = vec![Span::raw(format!("{}    └─ ", indent))];
+
+            if live_edit {
+                val_spans.push(Span::styled(app.input.value().to_string(), value_style));
+            } else if is_missing {
+                // Nothing has been accepted yet, so show what is on offer.
+                let offer = var.template_value.as_deref().unwrap_or("");
+                val_spans.push(Span::styled(
+                    format!("({})", offer),
+                    Style::default()
+                        .fg(theme.fg_dimmed())
+                        .add_modifier(Modifier::ITALIC),
+                ));
+            } else {
+                val_spans.push(Span::styled(
+                    var.value.as_deref().unwrap_or("").to_string(),
+                    value_style,
+                ));
+
+                if let Some(default) = &var.template_value
+                    && Some(default) != var.value.as_ref()
+                {
+                    let style = if is_selected {
+                        Style::default().fg(theme.bg_normal()).add_modifier(Modifier::DIM)
+                    } else {
+                        Style::default()
+                            .fg(theme.fg_dimmed())
+                            .add_modifier(Modifier::ITALIC)
+                    };
+                    val_spans.push(Span::styled(format!("  [default: {}]", default), style));
+                }
+            }
+
+            ListItem::new(vec![Line::from(key_spans), Line::from(val_spans)]).style(item_style)
         })
         .collect();
 
@@ -170,144 +196,505 @@ pub fn draw(f: &mut Frame, app: &mut App, config: &Config) {
         Block::default()
             .borders(Borders::ALL)
             .border_type(BorderType::Rounded)
-            .title(" Config Variables ")
-            .title_style(
-                Style::default()
-                    .fg(theme.fg_accent())
-                    .add_modifier(Modifier::BOLD),
-            )
+            .title(file_header(app, theme))
+            .title(counts_header(app, theme))
             .border_style(Style::default().fg(theme.border_normal())),
     );
 
     let mut state = ListState::default();
-    state.select(Some(app.selected));
-    f.render_stateful_widget(list, chunks[1], &mut state);
+    state.select(Some(app.visible_position()));
+    f.render_stateful_widget(list, area, &mut state);
+}
 
-    // Render the focused input area.
-    let current_var = app.vars.get(app.selected);
-    let mut input_title = " Input ".to_string();
-    let mut extra_info = String::new();
+/// `output ← template`, so the user always knows what `:w` will touch.
+fn file_header<'a>(app: &'a App, theme: &ThemeConfig) -> Line<'a> {
+    let mut spans = vec![
+        Span::raw(" "),
+        Span::styled(
+            display_path(&app.output_path),
+            Style::default()
+                .fg(theme.fg_accent())
+                .add_modifier(Modifier::BOLD),
+        ),
+    ];
 
-    if let Some(var) = current_var {
-        if var.is_group {
-            input_title = format!(" Group: {} ", var.path);
-        } else {
-            input_title = format!(" Editing: {} ", var.path);
-            if let Some(t_val) = &var.template_value {
-                extra_info = format!("  [Template: {}]", t_val);
-            }
-        }
+    if let Some(template) = &app.template_path {
+        spans.push(Span::styled(" ← ", Style::default().fg(theme.fg_dimmed())));
+        spans.push(Span::styled(
+            display_path(template),
+            Style::default().fg(theme.fg_dimmed()),
+        ));
     }
+    spans.push(Span::raw(" "));
+    Line::from(spans)
+}
 
-    let input_border_color = match app.mode {
-        Mode::Insert => theme.border_active(),
-        Mode::Normal | Mode::Search => theme.border_normal(),
+/// Right-aligned summary of what still needs attention.
+fn counts_header<'a>(app: &App, theme: &ThemeConfig) -> Line<'a> {
+    let mut spans = vec![Span::raw(" ")];
+    let missing = app.missing_count();
+    let changed = app.changed_count();
+
+    if missing > 0 {
+        spans.push(Span::styled(
+            format!("{} missing ", missing),
+            Style::default()
+                .fg(theme.fg_warning())
+                .add_modifier(Modifier::BOLD),
+        ));
+    }
+    if changed > 0 {
+        spans.push(Span::styled(
+            format!("{} changed ", changed),
+            Style::default().fg(theme.fg_modified()),
+        ));
+    }
+    if missing == 0 && changed == 0 {
+        spans.push(Span::styled(
+            "in sync ",
+            Style::default().fg(theme.fg_dimmed()),
+        ));
+    }
+    Line::from(spans).alignment(Alignment::Right)
+}
+
+/// Shortens a path to its file name when it sits in the working directory.
+fn display_path(path: &std::path::Path) -> String {
+    let cwd = std::env::current_dir().unwrap_or_default();
+    path.strip_prefix(&cwd)
+        .unwrap_or(path)
+        .display()
+        .to_string()
+}
+
+/// The pane under the tree: a read-only summary in Normal mode, the editor otherwise.
+fn draw_detail(f: &mut Frame, area: Rect, app: &App, theme: &ThemeConfig) {
+    let editing = matches!(app.mode, Mode::Insert | Mode::InsertKey);
+    let current = app.vars.get(app.selected);
+
+    let title = match (current, app.mode) {
+        (Some(var), Mode::InsertKey) => format!(" Rename: {} ", var.path_string()),
+        (Some(var), Mode::Insert) => format!(" Editing: {} ", var.path_string()),
+        (Some(var), _) => format!(" {} ", var.path_string()),
+        (None, _) => " Empty ".to_string(),
     };
 
-    let input_text = app.input.value();
-    let cursor_pos = app.input.visual_cursor();
+    let body: Line = if editing {
+        Line::from(app.input.value().to_string())
+    } else if let Some(var) = current {
+        // Read-only facts about the selection, so the box never looks like a
+        // second place to type when it is not one.
+        let mut spans = vec![Span::styled(
+            match var.value_type {
+                ValueType::String => "text",
+                ValueType::Number => "number",
+                ValueType::Bool => "bool",
+                ValueType::Null => if var.is_group { "group" } else { "empty" },
+            },
+            Style::default().fg(theme.fg_accent()),
+        )];
 
-    // Show template value in normal mode if it differs
-    let display_text = if let Some(var) = current_var {
-        if var.is_group {
-            "<group>".to_string()
-        } else if matches!(app.mode, Mode::Normal) {
-            format!("{}{}", input_text, extra_info)
-        } else {
-            input_text.to_string()
+        let status = match var.status {
+            ItemStatus::MissingFromActive => Some(("not in output yet", theme.fg_warning())),
+            ItemStatus::Overridden => Some(("differs from template", theme.fg_dimmed())),
+            ItemStatus::Present => None,
+        };
+        if let Some((label, color)) = status {
+            spans.push(Span::raw("  ·  "));
+            spans.push(Span::styled(label, Style::default().fg(color)));
         }
+        if var.dirty {
+            spans.push(Span::raw("  ·  "));
+            spans.push(Span::styled(
+                "edited",
+                Style::default().fg(theme.fg_modified()),
+            ));
+        }
+        if let Some(default) = &var.template_value {
+            spans.push(Span::raw("  ·  "));
+            spans.push(Span::styled(
+                format!("template: {}", default),
+                Style::default().fg(theme.fg_dimmed()),
+            ));
+        }
+        Line::from(spans)
     } else {
-        input_text.to_string()
+        Line::from(Span::styled(
+            "no entries — press o to add one",
+            Style::default().fg(theme.fg_dimmed()),
+        ))
     };
 
-    let input = Paragraph::new(display_text)
+    let border_color = if editing {
+        theme.border_active()
+    } else {
+        theme.border_normal()
+    };
+
+    let pane = Paragraph::new(body)
         .style(Style::default().fg(theme.fg_normal()))
         .block(
             Block::default()
                 .borders(Borders::ALL)
                 .border_type(BorderType::Rounded)
-                .title(input_title)
+                .title(title)
                 .title_style(
                     Style::default()
-                        .fg(theme.fg_accent()) // Make title pop
+                        .fg(theme.fg_accent())
                         .add_modifier(Modifier::BOLD),
                 )
-                .border_style(Style::default().fg(input_border_color)),
+                .border_style(Style::default().fg(border_color)),
         );
-    f.render_widget(input, chunks[2]);
+    f.render_widget(pane, area);
 
-    // Position the terminal cursor correctly when in Insert mode.
-    if let Mode::Insert = app.mode {
+    if editing {
         f.set_cursor_position(ratatui::layout::Position::new(
-            chunks[2].x + 1 + cursor_pos as u16,
-            chunks[2].y + 1,
+            area.x + 1 + app.input.visual_cursor() as u16,
+            area.y + 1,
         ));
     }
+}
 
-    // Render the modern pill-style status bar.
-    let (mode_str, mode_style) = match app.mode {
-        Mode::Normal => (
-            " NORMAL ",
-            Style::default()
-                .bg(theme.bg_highlight())
-                .fg(theme.bg_normal())
-                .add_modifier(Modifier::BOLD),
-        ),
-        Mode::Insert => (
-            " INSERT ",
-            Style::default()
-                .bg(theme.bg_active())
-                .fg(theme.bg_normal())
-                .add_modifier(Modifier::BOLD),
-        ),
-        Mode::Search => (
-            " SEARCH ",
-            Style::default()
-                .bg(theme.bg_search())
-                .fg(theme.bg_normal())
-                .add_modifier(Modifier::BOLD),
-        ),
+/// Mode indicator plus either the active prompt, a transient message, or hints.
+fn draw_status(f: &mut Frame, area: Rect, app: &App, config: &Config) {
+    let theme = &config.theme;
+    let (mode_str, mode_bg) = match app.mode {
+        Mode::Normal => (" NORMAL ", theme.bg_highlight()),
+        Mode::Insert => (" INSERT ", theme.bg_active()),
+        Mode::InsertKey => (" RENAME ", theme.bg_active()),
+        Mode::Search => (" SEARCH ", theme.bg_search()),
+        Mode::Help => (" HELP ", theme.bg_search()),
     };
 
-    let status_msg = if let Some(msg) = &app.status_message {
-        msg.clone()
-    } else {
-        let kb = &config.keybinds;
-        match app.mode {
-            Mode::Normal => {
-                let mut parts = vec![
-                    format!("{}/{} move", kb.down, kb.up),
-                    format!("{}/{} jump", kb.jump_top, kb.jump_bottom),
-                    format!("{} search", kb.search),
-                ];
-                if !app.selected_is_group() {
-                    parts.push(format!("{}/{}/{} edit", kb.edit, kb.edit_append, kb.edit_substitute));
-                }
-                if app.selected_is_missing() {
-                    parts.push(format!("{} add", "a")); // 'a' is currently hardcoded in runner
-                }
-                if app.selected_is_array() {
-                    parts.push(format!("{}/{} array", kb.append_item, kb.prepend_item));
-                }
-                parts.push(format!("{} del", kb.delete_item));
-                parts.push(format!("{} undo", kb.undo));
-                parts.push(format!("{} save", kb.save));
-                parts.push(format!("{} quit", kb.quit));
-                parts.join(" · ")
-            }
-            Mode::Insert => "Esc normal · Enter commit".to_string(),
-            Mode::Search => "Esc normal · type to filter".to_string(),
-        }
-    };
+    let message = app
+        .prompt
+        .clone()
+        .or_else(|| app.status_text().map(str::to_string))
+        .unwrap_or_else(|| hints(app, config));
 
     let status_line = Line::from(vec![
-        Span::styled(mode_str, mode_style),
         Span::styled(
-            format!(" {} ", status_msg),
-            Style::default().bg(theme.border_normal()).fg(theme.fg_normal()),
+            mode_str,
+            Style::default()
+                .bg(mode_bg)
+                .fg(theme.bg_normal())
+                .add_modifier(Modifier::BOLD),
+        ),
+        Span::styled(
+            format!(" {} ", message),
+            Style::default()
+                .bg(theme.border_normal())
+                .fg(theme.fg_normal()),
         ),
     ]);
 
-    let status = Paragraph::new(status_line).style(Style::default().bg(theme.border_normal()));
-    f.render_widget(status, chunks[4]);
+    f.render_widget(
+        Paragraph::new(status_line).style(Style::default().bg(theme.border_normal())),
+        area,
+    );
 }
+
+/// A handful of hints that fit on one line; the rest lives behind the help key.
+fn hints(app: &App, config: &Config) -> String {
+    let kb = &config.keybinds;
+    let help = format!("{} help", kb.key_for(Action::Help));
+
+    match app.mode {
+        Mode::Insert => "Esc cancel · Enter commit".to_string(),
+        Mode::InsertKey => "Esc cancel · Enter rename".to_string(),
+        Mode::Search => match app.match_progress() {
+            Some((_, 0)) => "no matches · Esc cancel".to_string(),
+            Some((cur, total)) => format!("match {}/{} · Enter keep · Esc cancel", cur, total),
+            None => "type to search keys and values · Esc cancel".to_string(),
+        },
+        Mode::Help => format!(
+            "{}/{} scroll · any other key closes",
+            kb.key_for(Action::Down),
+            kb.key_for(Action::Up)
+        ),
+        Mode::Normal => {
+            let mut parts = vec![format!("{}/{} move", kb.key_for(Action::Down), kb.key_for(Action::Up))];
+
+            if app.selected_is_missing() {
+                parts.push(format!("{} take default", kb.key_for(Action::AddMissing)));
+            } else if app.selected_is_group() {
+                parts.push(format!("{} fold", kb.key_for(Action::ToggleFold)));
+                parts.push(format!("{} rename", kb.key_for(Action::Rename)));
+            } else {
+                parts.push(format!("{} edit", kb.key_for(Action::Edit)));
+                parts.push(format!("{} rename", kb.key_for(Action::Rename)));
+            }
+
+            parts.push(format!("{} search", kb.key_for(Action::Search)));
+            parts.push(format!("{} save", kb.save));
+            parts.push(help);
+            parts.join(" · ")
+        }
+    }
+}
+
+/// One line of the help overlay.
+enum HelpRow {
+    Section(&'static str),
+    Bind(String, &'static str),
+}
+
+/// Builds the full reference: every configured binding, then the `:` commands.
+fn help_rows(config: &Config) -> Vec<HelpRow> {
+    let kb = &config.keybinds;
+    let mut rows = Vec::new();
+    let mut section = "";
+
+    for (spec, action) in kb.table() {
+        if action.section() != section {
+            section = action.section();
+            rows.push(HelpRow::Section(section));
+        }
+        rows.push(HelpRow::Bind(spec.to_string(), action.label()));
+    }
+
+    rows.push(HelpRow::Section("Commands"));
+    rows.push(HelpRow::Bind(kb.save.clone(), "write the output file"));
+    rows.push(HelpRow::Bind(format!("{} <path>", kb.save), "write somewhere else"));
+    rows.push(HelpRow::Bind(kb.quit.clone(), "quit (refused if unsaved)"));
+    rows.push(HelpRow::Bind(":q!".to_string(), "quit, discarding changes"));
+    rows.push(HelpRow::Bind(":wq".to_string(), "write and quit"));
+    rows
+}
+
+/// Renders one help row into styled spans of a fixed width.
+fn help_spans<'a>(row: Option<&'a HelpRow>, theme: &ThemeConfig, width: usize) -> Vec<Span<'a>> {
+    let key_width = 11;
+    match row {
+        Some(HelpRow::Section(name)) => vec![Span::styled(
+            format!("{:<width$}", name, width = width),
+            Style::default()
+                .fg(theme.fg_accent())
+                .add_modifier(Modifier::BOLD),
+        )],
+        Some(HelpRow::Bind(spec, label)) => {
+            let label_width = width.saturating_sub(key_width + 2);
+            vec![
+                Span::styled(
+                    format!("  {:<width$}", spec, width = key_width),
+                    Style::default().fg(theme.bg_search()),
+                ),
+                Span::styled(
+                    format!("{:<width$}", truncate(label, label_width), width = label_width),
+                    Style::default().fg(theme.fg_normal()),
+                ),
+            ]
+        }
+        None => vec![Span::raw(" ".repeat(width))],
+    }
+}
+
+/// Clips a label to the space available, with an ellipsis when it does not fit.
+fn truncate(text: &str, width: usize) -> String {
+    if text.chars().count() <= width {
+        return text.to_string();
+    }
+    text.chars().take(width.saturating_sub(1)).collect::<String>() + "…"
+}
+
+/// Full keybinding reference, laid out in two columns so it fits a short terminal.
+fn draw_help(f: &mut Frame, area: Rect, config: &Config, scroll_offset: usize) {
+    let theme = &config.theme;
+    let rows = help_rows(config);
+
+    // Break into two columns at the section boundary nearest the midpoint, so a
+    // group of related keys is never split across the gutter.
+    let midpoint = rows.len().div_ceil(2);
+    let split = (0..rows.len())
+        .filter(|&i| matches!(rows[i], HelpRow::Section(_)))
+        .min_by_key(|&i| i.abs_diff(midpoint))
+        .unwrap_or(midpoint);
+
+    let (left, right) = rows.split_at(split);
+    let column_width = 36usize;
+    let gutter = 2usize;
+
+    let lines: Vec<Line> = (0..left.len().max(right.len()))
+        .map(|i| {
+            let mut spans = help_spans(left.get(i), theme, column_width);
+            spans.push(Span::raw(" ".repeat(gutter)));
+            spans.extend(help_spans(right.get(i), theme, column_width));
+            Line::from(spans)
+        })
+        .collect();
+
+    let popup = centered_rect(
+        (column_width * 2 + gutter + 2) as u16,
+        area,
+        lines.len() as u16 + 2,
+    );
+    // On a terminal too short for the whole reference, scroll rather than clip
+    // silently: a key the user cannot see is a key they do not have.
+    let rows_shown = popup.height.saturating_sub(2) as usize;
+    let max_scroll = lines.len().saturating_sub(rows_shown);
+    let scroll = scroll_offset.min(max_scroll);
+    let title = if max_scroll > 0 {
+        format!(" Keys — {}/{} ", (scroll + rows_shown).min(lines.len()), lines.len())
+    } else {
+        " Keys ".to_string()
+    };
+    f.render_widget(Clear, popup);
+    f.render_widget(
+        Paragraph::new(lines)
+            .scroll((scroll as u16, 0))
+            .style(Style::default().bg(theme.bg_normal()).fg(theme.fg_normal()))
+            .block(
+                Block::default()
+                    .borders(Borders::ALL)
+                    .border_type(BorderType::Rounded)
+                    .title(title)
+                    .title_style(
+                        Style::default()
+                            .fg(theme.fg_accent())
+                            .add_modifier(Modifier::BOLD),
+                    )
+                    .border_style(Style::default().fg(theme.border_active())),
+            ),
+        popup,
+    );
+}
+
+/// Centers a popup of the given size inside `area`, clamped to what fits.
+fn centered_rect(width: u16, area: Rect, height: u16) -> Rect {
+    let w = width.min(area.width.saturating_sub(2)).max(1);
+    let h = height.min(area.height.saturating_sub(2)).max(1);
+    Rect {
+        x: area.x + (area.width.saturating_sub(w)) / 2,
+        y: area.y + (area.height.saturating_sub(h)) / 2,
+        width: w,
+        height: h,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::format::{ConfigItem, PathSegment};
+    use ratatui::Terminal;
+    use ratatui::backend::TestBackend;
+    use std::path::PathBuf;
+
+    /// Renders one frame and returns it as plain text lines.
+    fn render(app: &mut App, config: &Config) -> Vec<String> {
+        let mut terminal = Terminal::new(TestBackend::new(80, 20)).unwrap();
+        terminal.draw(|f| draw(f, app, config)).unwrap();
+        let buffer = terminal.backend().buffer().clone();
+        (0..buffer.area.height)
+            .map(|y| {
+                (0..buffer.area.width)
+                    .map(|x| buffer[(x, y)].symbol())
+                    .collect::<String>()
+                    .trim_end()
+                    .to_string()
+            })
+            .collect()
+    }
+
+    fn sample_app() -> App {
+        let mut token = ConfigItem::leaf(
+            "API_KEY",
+            vec![PathSegment::Key("API_KEY".to_string())],
+            "",
+            0,
+            ValueType::String,
+        );
+        token.status = ItemStatus::MissingFromActive;
+        token.value = None;
+        token.template_value = Some("changeme".to_string());
+
+        App::new(
+            vec![
+                ConfigItem::leaf(
+                    "DB_HOST",
+                    vec![PathSegment::Key("DB_HOST".to_string())],
+                    "prod.internal",
+                    0,
+                    ValueType::String,
+                ),
+                token,
+            ],
+            PathBuf::from(".env"),
+            Some(PathBuf::from(".env.example")),
+        )
+    }
+
+    #[test]
+    fn header_names_the_files_and_counts_the_work() {
+        let config = Config::default();
+        let mut app = sample_app();
+        let frame = render(&mut app, &config).join("\n");
+
+        assert!(frame.contains(".env"), "{frame}");
+        assert!(frame.contains(".env.example"), "{frame}");
+        assert!(frame.contains("1 missing"), "{frame}");
+        assert!(frame.contains("(missing)"), "{frame}");
+    }
+
+    #[test]
+    fn status_bar_offers_the_help_key_and_fits_the_terminal() {
+        let config = Config::default();
+        let mut app = sample_app();
+        app.selected = 1; // the entry that is missing from the output
+        let frame = render(&mut app, &config);
+        let status = frame.last().unwrap();
+
+        assert!(status.contains("NORMAL"), "{status}");
+        assert!(status.contains("? help"), "{status}");
+        // The hint line must survive an 80-column terminal without truncation.
+        assert!(status.chars().count() <= 80, "{status}");
+        // On a missing entry the offer to take the default is what matters.
+        assert!(status.contains("take default"), "{status}");
+    }
+
+    #[test]
+    fn help_overlay_lists_bindings() {
+        let config = Config::default();
+        let mut app = sample_app();
+        app.toggle_help();
+        let frame = render(&mut app, &config).join("\n");
+
+        assert!(frame.contains("Navigate"), "{frame}");
+        assert!(frame.contains("Template"), "{frame}");
+        assert!(frame.contains("take template default"), "{frame}");
+        assert!(frame.contains("History"), "{frame}");
+        assert!(frame.contains("Session"), "{frame}");
+    }
+
+    #[test]
+    fn help_overlay_scrolls_when_the_terminal_is_short() {
+        let config = Config::default();
+        let mut app = sample_app();
+        app.toggle_help();
+
+        // Too short to show everything at once: say so, and let the rest be reached.
+        let frame = render(&mut app, &config).join("\n");
+        assert!(frame.contains("Keys — "), "{frame}");
+        assert!(!frame.contains(":q!"), "{frame}");
+
+        for _ in 0..10 {
+            app.scroll_help(true);
+        }
+        let frame = render(&mut app, &config).join("\n");
+        assert!(frame.contains(":q!"), "{frame}");
+    }
+
+    #[test]
+    fn transient_status_replaces_hints_then_gives_them_back() {
+        let config = Config::default();
+        let mut app = sample_app();
+        app.set_status("Saved to .env");
+
+        let frame = render(&mut app, &config);
+        assert!(frame.last().unwrap().contains("Saved to .env"));
+
+        app.clear_status();
+        let frame = render(&mut app, &config);
+        assert!(frame.last().unwrap().contains("? help"));
+    }
+}
+

src/undo.rs

@@ -0,0 +1,302 @@
+use crate::format::ConfigItem;
+use std::collections::HashMap;
+
+/// Represents a single snapshot of the application state for undo/redo purposes.
+pub struct EditAction {
+    /// The complete list of configuration items at the time of the action.
+    pub state: Vec<ConfigItem>,
+    /// The index of the item that was selected during this action.
+    pub selected: usize,
+}
+
+/// A node in the undo tree, representing a point in the application's history.
+pub struct UndoNode {
+    /// The state data captured at this history point.
+    pub action: EditAction,
+    /// ID of the parent node (previous state). Root node has None.
+    pub parent: Option<usize>,
+    /// IDs of all states that branched off from this one.
+    pub children: Vec<usize>,
+}
+
+/// A non-linear undo/redo system that tracks history as a branching tree.
+///
+/// Unlike a simple stack, an UndoTree allows users to undo several steps, 
+/// make a new change (creating a branch), and still navigate through 
+/// the most recent history path.
+pub struct UndoTree {
+    /// Map of node IDs to their respective history nodes.
+    nodes: HashMap<usize, UndoNode>,
+    /// The ID of the node representing the current application state.
+    current_node: usize,
+    /// Counter for assigning unique IDs to new nodes.
+    next_id: usize,
+    /// Tracks the most recently active branch for each node.
+    /// This allows the 'redo' operation to follow the path the user 
+    /// actually took when multiple branches exist.
+    latest_branch: HashMap<usize, usize>,
+}
+
+impl UndoTree {
+    /// Creates a new undo tree initialized with the starting application state.
+    pub fn new(initial_state: Vec<ConfigItem>, initial_selected: usize) -> Self {
+        let root_id = 0;
+        let root_node = UndoNode {
+            action: EditAction {
+                state: initial_state,
+                selected: initial_selected,
+            },
+            parent: None,
+            children: Vec::new(),
+        };
+
+        let mut nodes = HashMap::new();
+        nodes.insert(root_id, root_node);
+
+        Self {
+            nodes,
+            current_node: root_id,
+            next_id: 1,
+            latest_branch: HashMap::new(),
+        }
+    }
+
+    /// Pushes a new state onto the tree, branching off from the current node.
+    ///
+    /// This creates a new child node for the current position and updates
+    /// the branch tracking to ensure this new path is preferred during redo.
+    pub fn push(&mut self, state: Vec<ConfigItem>, selected: usize) {
+        let new_id = self.next_id;
+        self.next_id += 1;
+
+        let new_node = UndoNode {
+            action: EditAction { state, selected },
+            parent: Some(self.current_node),
+            children: Vec::new(),
+        };
+
+        // Add to nodes
+        self.nodes.insert(new_id, new_node);
+
+        // Update parent's children
+        if let Some(parent_node) = self.nodes.get_mut(&self.current_node) {
+            parent_node.children.push(new_id);
+        }
+
+        // Record this as the latest branch for the parent
+        self.latest_branch.insert(self.current_node, new_id);
+
+        // Move current pointer
+        self.current_node = new_id;
+    }
+
+    /// Moves the current pointer back to the parent node and returns the previous state.
+    /// Returns None if the current node is the root (no more history to undo).
+    pub fn undo(&mut self) -> Option<&EditAction> {
+        if let Some(current) = self.nodes.get(&self.current_node)
+            && let Some(parent_id) = current.parent {
+                self.current_node = parent_id;
+                return self.nodes.get(&parent_id).map(|n| &n.action);
+            }
+        None
+    }
+
+    /// Moves the current pointer forward to the latest child branch and returns the state.
+    ///
+    /// Redo follows the `latest_branch` map to decide which path to take 
+    /// if multiple branches exist. If no branch is recorded, it defaults to the 
+    /// most recently created child.
+    pub fn redo(&mut self) -> Option<&EditAction> {
+        if let Some(next_id) = self.latest_branch.get(&self.current_node).copied() {
+            self.current_node = next_id;
+            return self.nodes.get(&next_id).map(|n| &n.action);
+        } else {
+            // Fallback: if there is no recorded latest branch but there are children
+            let current_id = self.current_node;
+            if let Some(current) = self.nodes.get(&current_id)
+                && let Some(&first_child_id) = current.children.last() {
+                    self.current_node = first_child_id;
+                    self.latest_branch.insert(current_id, first_child_id);
+                    return self.nodes.get(&first_child_id).map(|n| &n.action);
+                }
+        }
+        None
+    }
+
+    /// Discards the current node, moving back to its parent.
+    ///
+    /// Used when an action is rolled back before the user ever saw it as a
+    /// distinct step, such as a newly added item abandoned at the rename prompt.
+    pub fn pop(&mut self) {
+        let Some(current) = self.nodes.get(&self.current_node) else { return };
+        let Some(parent_id) = current.parent else { return };
+
+        let removed = self.current_node;
+        self.nodes.remove(&removed);
+        if let Some(parent) = self.nodes.get_mut(&parent_id) {
+            parent.children.retain(|&c| c != removed);
+        }
+        if self.latest_branch.get(&parent_id) == Some(&removed) {
+            match self.nodes.get(&parent_id).and_then(|p| p.children.last()) {
+                Some(&fallback) => {
+                    self.latest_branch.insert(parent_id, fallback);
+                }
+                None => {
+                    self.latest_branch.remove(&parent_id);
+                }
+            }
+        }
+        self.latest_branch.remove(&removed);
+        self.current_node = parent_id;
+    }
+
+    /// Which forward branch redo would take, as `(index, total)`, both 1-based.
+    ///
+    /// `None` when the current node has no children at all.
+    pub fn branch_position(&self) -> Option<(usize, usize)> {
+        let current = self.nodes.get(&self.current_node)?;
+        if current.children.is_empty() {
+            return None;
+        }
+        let preferred = self
+            .latest_branch
+            .get(&self.current_node)
+            .copied()
+            .or_else(|| current.children.last().copied())?;
+        let index = current
+            .children
+            .iter()
+            .position(|&c| c == preferred)
+            .unwrap_or(0);
+        Some((index + 1, current.children.len()))
+    }
+
+    /// Picks a different forward branch at the current point.
+    ///
+    /// Returns the new `(index, total)` position, or `None` when there is no
+    /// alternative history to choose from.
+    pub fn cycle_branch(&mut self, forward: bool) -> Option<(usize, usize)> {
+        let (index, total) = self.branch_position()?;
+        if total < 2 {
+            return None;
+        }
+        let next = if forward {
+            index % total
+        } else {
+            (index + total - 2) % total
+        };
+        let child = self.nodes.get(&self.current_node)?.children[next];
+        self.latest_branch.insert(self.current_node, child);
+        Some((next + 1, total))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::format::ValueType;
+
+    fn dummy_item(key: &str) -> ConfigItem {
+        ConfigItem::leaf(key, vec![], key, 0, ValueType::String)
+    }
+
+    #[test]
+    fn test_undo_redo_tree() {
+        let state1 = vec![dummy_item("A")];
+        let mut tree = UndoTree::new(state1.clone(), 0);
+
+        // Push state 2
+        let state2 = vec![dummy_item("B")];
+        tree.push(state2.clone(), 1);
+
+        // Push state 3
+        let state3 = vec![dummy_item("C")];
+        tree.push(state3.clone(), 2);
+
+        // Undo -> State 2
+        let action = tree.undo().unwrap();
+        assert_eq!(action.state[0].key, "B");
+        assert_eq!(action.selected, 1);
+
+        // Undo -> State 1
+        let action = tree.undo().unwrap();
+        assert_eq!(action.state[0].key, "A");
+        assert_eq!(action.selected, 0);
+
+        // Undo again -> None (already at root)
+        assert!(tree.undo().is_none());
+
+        // Redo -> State 2
+        let action = tree.redo().unwrap();
+        assert_eq!(action.state[0].key, "B");
+        assert_eq!(action.selected, 1);
+
+        // Redo -> State 3
+        let action = tree.redo().unwrap();
+        assert_eq!(action.state[0].key, "C");
+        assert_eq!(action.selected, 2);
+
+        // Branching: Undo twice to State 1
+        tree.undo();
+        tree.undo();
+        
+        // Push State 4 (from State 1)
+        let state4 = vec![dummy_item("D")];
+        tree.push(state4.clone(), 3);
+
+        // Undo -> State 1
+        let action = tree.undo().unwrap();
+        assert_eq!(action.state[0].key, "A");
+
+        // Redo -> State 4 (follows latest branch D, not old branch B)
+        let action = tree.redo().unwrap();
+        assert_eq!(action.state[0].key, "D");
+    }
+
+    #[test]
+    fn test_redo_fallback_fix() {
+        let state1 = vec![dummy_item("A")];
+        let mut tree = UndoTree::new(state1.clone(), 0);
+
+        let state2 = vec![dummy_item("B")];
+        tree.push(state2.clone(), 1);
+
+        tree.undo();
+        // Redo should move to state 2
+        let action = tree.redo().unwrap();
+        assert_eq!(action.state[0].key, "B");
+
+        // Calling redo again should NOT change the current node or returned action
+        // (since it's already at the latest child)
+        assert!(tree.redo().is_none());
+    }
+
+    #[test]
+    fn test_pop_discards_current_state() {
+        let mut tree = UndoTree::new(vec![dummy_item("A")], 0);
+        tree.push(vec![dummy_item("B")], 1);
+
+        tree.pop();
+
+        // Back at A, with nothing left to redo into.
+        assert!(tree.redo().is_none());
+        assert!(tree.undo().is_none());
+    }
+
+    #[test]
+    fn test_branch_cycling() {
+        let mut tree = UndoTree::new(vec![dummy_item("A")], 0);
+        tree.push(vec![dummy_item("B")], 0);
+        tree.undo();
+        tree.push(vec![dummy_item("C")], 0);
+        tree.undo();
+
+        // Two futures hang off the root; redo prefers the newest.
+        assert_eq!(tree.branch_position(), Some((2, 2)));
+        assert_eq!(tree.redo().unwrap().state[0].key, "C");
+
+        tree.undo();
+        tree.cycle_branch(true);
+        assert_eq!(tree.redo().unwrap().state[0].key, "B");
+    }
+}