Fix continuous page seeking

src/viewer.rs

@@ -243,6 +243,7 @@ impl ViewerWindow {
             navigation_message,
             navigation_message_label,
             navigation_message_generation: Rc::new(Cell::new(0)),
+            continuous_programmatic_scroll: Cell::new(false),
             state: RefCell::new(ViewerState {
                 pages: Vec::new(),
                 anchor: 0,
@@ -333,6 +334,7 @@ struct ViewerController {
     navigation_message: gtk::Box,
     navigation_message_label: gtk::Label,
     navigation_message_generation: Rc<Cell<u64>>,
+    continuous_programmatic_scroll: Cell<bool>,
     state: RefCell<ViewerState>,
 }
 
@@ -948,13 +950,16 @@ impl ViewerController {
     }
 
     fn continuous_scrolled(self: &Rc<Self>) {
-        if self.view_mode() != ViewMode::Continuous || self.state.borrow().pages.is_empty() {
+        let pages_empty = self.state.borrow().pages.is_empty();
+        if self.view_mode() != ViewMode::Continuous || pages_empty {
             return;
         }
         let visible = self.visible_indices();
         let new_anchor = visible.first().copied();
-        let changed = new_anchor.is_some_and(|index| index != self.state.borrow().anchor);
-        if let Some(index) = new_anchor {
+        let update_anchor = !self.continuous_programmatic_scroll.get();
+        let changed =
+            update_anchor && new_anchor.is_some_and(|index| index != self.state.borrow().anchor);
+        if let Some(index) = new_anchor.filter(|_| update_anchor) {
             self.state.borrow_mut().anchor = index;
         }
         self.render_continuous(&visible);
@@ -1014,16 +1019,28 @@ impl ViewerController {
     }
 
     fn scroll_to_anchor(&self) {
+        let frame = {
             let state = self.state.borrow();
             let Some(row) = state.continuous_rows.get(state.anchor) else {
                 return;
             };
-        let Some(bounds) = row.frame.compute_bounds(&self.continuous_box) else {
+            row.frame.clone()
+        };
+        let Some(bounds) = frame.compute_bounds(&self.continuous_box) else {
             return;
         };
         let adjustment = self.continuous_scrolled.vadjustment();
         let maximum = (adjustment.upper() - adjustment.page_size()).max(adjustment.lower());
-        adjustment.set_value(f64::from(bounds.y()).clamp(adjustment.lower(), maximum));
+        let target = centered_scroll_value(
+            f64::from(bounds.y()),
+            f64::from(bounds.height()),
+            adjustment.page_size(),
+            adjustment.lower(),
+            maximum,
+        );
+        self.continuous_programmatic_scroll.set(true);
+        adjustment.set_value(target);
+        self.continuous_programmatic_scroll.set(false);
     }
 
     fn clear_continuous_paintables(&self) {
@@ -1629,6 +1646,16 @@ fn intersecting_rows(bounds: &[Option<(f64, f64)>], top: f64, bottom: f64) -> Ve
         .collect()
 }
 
+fn centered_scroll_value(
+    row_top: f64,
+    row_height: f64,
+    viewport_height: f64,
+    minimum: f64,
+    maximum: f64,
+) -> f64 {
+    (row_top + row_height / 2.0 - viewport_height / 2.0).clamp(minimum, maximum)
+}
+
 fn page_seek_index(value: f64, count: usize) -> Option<usize> {
     (count > 0).then(|| (value.round().max(0.0) as usize).min(count - 1))
 }
@@ -1673,6 +1700,19 @@ mod tests {
         assert_eq!(intersecting_rows(&bounds, 3000.0, 3800.0), [2]);
     }
 
+    #[test]
+    fn continuous_seek_centers_the_selected_row() {
+        assert_eq!(
+            centered_scroll_value(1_000.0, 800.0, 600.0, 0.0, 5_000.0),
+            1_100.0
+        );
+        assert_eq!(centered_scroll_value(0.0, 400.0, 800.0, 0.0, 5_000.0), 0.0);
+        assert_eq!(
+            centered_scroll_value(5_000.0, 800.0, 600.0, 0.0, 5_000.0),
+            5_000.0
+        );
+    }
+
     #[test]
     fn fast_loading_prefetches_a_larger_window() {
         assert_eq!(prefetch_window(&[10], 30, false), [10, 11, 12, 9]);