fix(gui): keep paging the timeline when zoomed out onto one screen

Zoomed all the way out, a 200-photo page fits on one screen. With
nothing to scroll, the scroll handler never asked for the next page and
the timeline stopped after the first rows. Ask for more whenever the
content no longer fills a screen and a half, also after a zoom step or
a resize, not only when a page lands.

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

@@ -1394,6 +1394,13 @@ pub(crate) fn wire_gallery(
         sync_float_heading(&ui_scroll);
         ui_scroll.gallery.scrubber.flash();
     });
+    // Zooming out or making the window taller can leave every loaded photo on
+    // one screen. Nothing scrolls then, so the scrolling above never asks for
+    // the next page: ask whenever the content shrinks under the viewport.
+    let ui_fill = ui.clone();
+    scroll
+        .vadjustment()
+        .connect_changed(move |_| fill_viewport(&ui_fill));
     wire_scrubber(ui);
     wire_drag_select(ui);
 
@@ -4090,17 +4097,32 @@ fn continue_jump(ui: &Rc<Ui>) {
 /// Load the next page when the timeline does not fill the window yet: with no
 /// scrollbar there is no scrolling to trigger it.
 fn fill_viewport(ui: &Rc<Ui>) {
+    if !ui.gallery.has_more.get() || ui.gallery.loading.get() {
+        return;
+    }
     let ui = ui.clone();
     glib::idle_add_local_once(move || {
         let Some(adj) = ui.gallery.list.vadjustment() else {
             return;
         };
-        if ui.gallery.has_more.get() && adj.upper() <= adj.page_size() + 1.0 {
+        let timeline = ui.gallery.content.visible_child_name().as_deref() == Some("timeline");
+        if timeline
+            && ui.gallery.has_more.get()
+            && adj.page_size() > 0.0
+            && viewport_unfilled(adj.upper(), adj.page_size())
+        {
             load_gallery(&ui, true);
         }
     });
 }
 
+/// Whether content `upper` px tall leaves a viewport `page_size` px tall short
+/// of a screen and a half to scroll through, the margin the scrolling pager
+/// keeps. Below it the pager would never fire.
+pub(crate) fn viewport_unfilled(upper: f64, page_size: f64) -> bool {
+    upper <= page_size * 2.5 + 1.0
+}
+
 /// Select every photo loaded so far (Ctrl+A).
 pub(crate) fn select_all_photos(ui: &Rc<Ui>) {
     set_selection_mode(ui, true);
@@ -4494,10 +4516,10 @@ pub(crate) fn play_external(path: &str) {
 #[cfg(test)]
 mod tests {
     use super::{
-        Grouping, ROW_DEFAULT, ROW_MAX, ROW_MIN, ROW_STEP, TILE_GAP, TILE_RATIO_MAX,
+        Grouping, PHOTOS_PAGE, ROW_DEFAULT, ROW_MAX, ROW_MIN, ROW_STEP, TILE_GAP, TILE_RATIO_MAX,
         TILE_RATIO_MIN, active_filters, copy_without_overwrite, empty_timeline_text, grouping_for,
         memory_years, month_index, month_range, plan_grid, plan_justified, section_heading,
-        tile_ratio,
+        tile_ratio, viewport_unfilled,
     };
     use pdfs_core::control::{PhotoItem, PhotoKind, PhotoMonth};
 
@@ -4620,6 +4642,18 @@ mod tests {
         std::fs::remove_dir_all(&dir).unwrap();
     }
 
+    /// A first page zoomed all the way out fits on one screen, with nothing
+    /// to scroll: the next page has to load without a scroll asking for it.
+    #[test]
+    fn a_page_zoomed_out_onto_one_screen_asks_for_more() {
+        let (columns, size) = plan_grid(1580, ROW_MIN);
+        let rows = PHOTOS_PAGE.div_ceil(columns) as i32;
+        let height = f64::from(rows * (size + TILE_GAP) + 40);
+        assert!(viewport_unfilled(height, 880.0));
+        assert!(viewport_unfilled(880.0, 880.0));
+        assert!(!viewport_unfilled(880.0 * 3.0, 880.0));
+    }
+
     #[test]
     fn zooming_in_puts_fewer_tiles_in_a_row() {
         assert!(plan_grid(1920, ROW_MIN).0 > plan_grid(1920, ROW_DEFAULT).0);