fix(gui): let the main window shrink to fit small screens

While signed in, the sidebar never collapsed. The window could not get
narrower than the sidebar plus the widest page, about 950 pixels, so on a
small or scaled screen the minimize, maximize and close buttons ended up
off-screen.

- The page stack is no longer homogeneous, so only the page on screen sets
  the minimum width.
- The selection bars of My files and Photos scroll sideways instead of
  holding the window at their full width, even while hidden.
- Below 760sp the sidebar and the page take turns, with a back button,
  and the My files search entry narrows. Picking a sidebar row opens the
  page.

Every page, in English and in German, now fits in a window 480 pixels
wide with its window buttons visible.

crates/pdfs-gui/src/app/main.rs

@@ -130,8 +130,12 @@ struct Ui {
     /// the page stack; [`sync_sidebar`] mirrors navigation that starts elsewhere.
     sidebar: gtk4::ListBox,
     /// The sidebar/content split. Collapsed while signed out, so the login page
-    /// owns the whole window and no destination is reachable without a session.
+    /// owns the whole window and no destination is reachable without a session,
+    /// and while the window is [`narrow`](Self::narrow).
     nav: adw::NavigationSplitView,
+    /// The window is too narrow for the sidebar beside a page, so the two take
+    /// turns. Set by the narrow breakpoint; [`refresh`] reads it.
+    narrow: Cell<bool>,
     /// Shared thumbnails for ordinary image files outside the Photos gallery.
     /// One cache and request queue serves Files, search, Shared and Trash, so
     /// the same image is downloaded and decoded only once.
@@ -321,7 +325,9 @@ fn build_window(app: &adw::Application) {
         tracing::warn!("cannot create the app's directories: {e}");
     }
 
-    let stack = adw::ViewStack::new();
+    // Not homogeneous: each page sets the window's minimum width only while it
+    // is shown, so the widest page does not hold every other one wide.
+    let stack = adw::ViewStack::builder().hhomogeneous(false).build();
     let (login_page, login_widgets) = build_login_page();
     let main_widgets = build_main_page();
     let (browser_page, browser_widgets) = build_browser_page();
@@ -368,6 +374,8 @@ fn build_window(app: &adw::Application) {
         .content(&content_page)
         .min_sidebar_width(200.0)
         .max_sidebar_width(240.0)
+        // A narrow window opens on the page, not the sidebar.
+        .show_content(true)
         .build();
 
     // Toasts float over everything, so a report from a background action reaches
@@ -386,6 +394,7 @@ fn build_window(app: &adw::Application) {
         mounted: RefCell::new(false),
         sidebar: sidebar_list.clone(),
         nav: split.clone(),
+        narrow: Cell::new(false),
         file_thumbs: FileThumbnailState::new(),
         login: login_widgets,
         status: StatusState {
@@ -735,6 +744,18 @@ fn build_window(app: &adw::Application) {
     );
     wide.add_setter(&ui.browser.split, "collapsed", Some(&false.to_value()));
     window.add_breakpoint(wide);
+    // A narrow window shows the sidebar and the page in turn, so every header
+    // bar keeps room for the window buttons. Signals rather than a setter:
+    // `refresh` decides the collapse while signed out, and would undo one.
+    let narrow = adw::Breakpoint::new(
+        adw::BreakpointCondition::parse("max-width: 760sp").expect("static breakpoint"),
+    );
+    narrow.add_setter(&ui.browser.search, "width-chars", Some(&8.to_value()));
+    let ui_narrow = ui.clone();
+    narrow.connect_apply(move |_| set_narrow(&ui_narrow, true));
+    let ui_wide = ui.clone();
+    narrow.connect_unapply(move |_| set_narrow(&ui_wide, false));
+    window.add_breakpoint(narrow);
     install_shortcuts(&ui, &window);
     install_window_actions(&ui, &window);
     install_launch_actions(&ui, app, &window);
@@ -748,6 +769,15 @@ fn build_window(app: &adw::Application) {
     window.present();
 }
 
+/// Follow the narrow breakpoint. While signed out the login page keeps the
+/// split collapsed whatever the width.
+fn set_narrow(ui: &Ui, narrow: bool) {
+    ui.narrow.set(narrow);
+    if ui.session.borrow().is_some() {
+        ui.nav.set_collapsed(narrow);
+    }
+}
+
 /// Save the window's size when it closes, and when the app quits with the
 /// window still open (Ctrl+Q, the tray's Quit), which never asks it to close.
 fn remember_window_state(ui: &Rc<Ui>, app: &adw::Application, window: &adw::ApplicationWindow) {
@@ -911,6 +941,18 @@ fn set_busy_spinners(visible: bool) {
     });
 }
 
+/// `row` in a frame that scrolls sideways once the window is too narrow for it,
+/// so a row of labelled buttons never sets the window's minimum width.
+fn scroll_sideways(row: &impl IsA<gtk4::Widget>) -> gtk4::ScrolledWindow {
+    gtk4::ScrolledWindow::builder()
+        .hscrollbar_policy(gtk4::PolicyType::Automatic)
+        .vscrollbar_policy(gtk4::PolicyType::Never)
+        .propagate_natural_width(true)
+        .propagate_natural_height(true)
+        .child(row)
+        .build()
+}
+
 /// A destination page's frame: its own header bar over the page content, the
 /// way GNOME apps lay a split view out. The title sits in the middle with the
 /// busy spinner beside it; the page packs its actions into the returned header
@@ -974,6 +1016,14 @@ fn wire_sidebar(ui: &Rc<Ui>) {
             ui_row.stack.set_visible_child_name(page);
         }
     });
+    // In a narrow window the sidebar covers the page, so picking a row, even
+    // the selected one, has to bring the page forward.
+    let ui_open = ui.clone();
+    ui.sidebar.connect_row_activated(move |_, _| {
+        if ui_open.nav.is_collapsed() {
+            ui_open.nav.set_show_content(true);
+        }
+    });
 }
 
 /// Re-fetch the visible page from the server, bypassing every layer of cache

crates/pdfs-gui/src/app/pages/browser.rs

@@ -376,7 +376,7 @@ pub(crate) fn build_browser_page() -> (gtk4::Widget, BrowserWidgets) {
     bulk_box.append(&bulk_clear);
     let bulk = gtk4::Revealer::builder()
         .transition_type(gtk4::RevealerTransitionType::SlideDown)
-        .child(&bulk_box)
+        .child(&scroll_sideways(&bulk_box))
         .build();
 
     // The status bar: contextual text (stretch 1), then the icon-size slider.

crates/pdfs-gui/src/app/pages/photos.rs

@@ -609,7 +609,7 @@ pub(crate) fn build_gallery_page() -> (gtk4::Widget, GalleryWidgets) {
     select_box.append(&select_done);
     let select_bar = gtk4::Revealer::builder()
         .transition_type(gtk4::RevealerTransitionType::SlideDown)
-        .child(&select_box)
+        .child(&scroll_sideways(&select_box))
         .build();
 
     // All / Photos / Videos / Raw filter. Linked toggles acting as one segmented

crates/pdfs-gui/src/app/pages/status.rs

@@ -860,7 +860,7 @@ pub(crate) fn refresh(ui: &Rc<Ui>) {
                 if ui.stack.visible_child_name().as_deref() == Some("login") {
                     ui.stack.set_visible_child_name("browser");
                 }
-                ui.nav.set_collapsed(false);
+                ui.nav.set_collapsed(ui.narrow.get());
                 if ui.status.account_name.label() != s.username {
                     ui.status.account_name.set_label(&s.username);
                     ui.status.account_name.set_tooltip_text(Some(&s.username));