fix: coalesce rapid viewport scrolling redraws

src/tui.rs

@@ -1433,6 +1433,8 @@ fn keys_ready(
     mut next: impl FnMut() -> io::Result<Option<Event>>,
 ) -> io::Result<Option<Event>> {
     let mode = session.editor().mode();
+    // Render before reversing/clamping a scroll or revealing a moved caret.
+    let scroll = session.editor().viewport_scroll_action(&input);
     for handled in 1..=64 {
         session.handle(input);
         if handled == 64 || session.editor().mode() != mode || session.editor().should_quit() {
@@ -1442,6 +1444,7 @@ fn keys_ready(
         if let Some(Event::Key(key)) = &pending
             && let Some(next) = translate(*key)
             && batch_key(session.editor(), key, &next)
+            && session.editor().viewport_scroll_action(&next) == scroll
         {
             input = next;
         } else {
@@ -17998,6 +18001,76 @@ end
         assert!(terminal.backend().cursor_visible());
     }
 
+    #[test]
+    fn queued_scrolls_coalesce_but_preserve_reversals_and_motion_barriers() {
+        let directory = Directory::new();
+        let text = (0..100)
+            .map(|row| format!("row-{row}\n"))
+            .collect::<String>();
+        for (first, repeated, barrier) in [
+            (
+                Input::Ctrl('u'),
+                'u',
+                KeyEvent::new(KeyCode::Char('d'), KeyModifiers::CONTROL),
+            ),
+            (
+                Input::Ctrl('d'),
+                'd',
+                KeyEvent::new(KeyCode::Home, KeyModifiers::NONE),
+            ),
+            (
+                Input::Down,
+                'j',
+                KeyEvent::new(KeyCode::Char('d'), KeyModifiers::CONTROL),
+            ),
+        ] {
+            let mut session =
+                EditorSession::open(Some(directory.file("rows.txt", &text)), vec![], None).unwrap();
+            let mut views = BufferViewports::default();
+            let mut terminal = Terminal::new(ratatui::backend::TestBackend::new(80, 12)).unwrap();
+            terminal
+                .draw(|frame| views.draw_session(frame, &mut session))
+                .unwrap();
+            let modifiers = if repeated == 'j' {
+                KeyModifiers::NONE
+            } else {
+                KeyModifiers::CONTROL
+            };
+            let mut events = std::iter::repeat_n(
+                Event::Key(KeyEvent::new(KeyCode::Char(repeated), modifiers)),
+                7,
+            )
+            .chain([Event::Key(barrier)]);
+            assert_eq!(
+                keys_ready(&mut session, first, || Ok(events.next())).unwrap(),
+                Some(Event::Key(barrier))
+            );
+            terminal
+                .draw(|frame| views.draw_session(frame, &mut session))
+                .unwrap();
+            if repeated == 'u' {
+                assert_eq!(views.offsets[&session.editor().buffer_id()].0, 0);
+                session.handle(Input::Ctrl('d'));
+                terminal
+                    .draw(|frame| views.draw_session(frame, &mut session))
+                    .unwrap();
+                assert_eq!(
+                    views.offsets[&session.editor().buffer_id()].0,
+                    usize::from(views.document.geometry.unwrap().1.height) / 2
+                );
+            } else if repeated == 'd' {
+                assert_eq!(views.offsets[&session.editor().buffer_id()].0, 40);
+                session.handle(Input::Home);
+                terminal
+                    .draw(|frame| views.draw_session(frame, &mut session))
+                    .unwrap();
+                assert_eq!(views.offsets[&session.editor().buffer_id()].0, 0);
+            } else {
+                assert_eq!(session.editor().cursor().row, 8);
+            }
+        }
+    }
+
     #[test]
     fn queued_navigation_preserves_counted_motions_visual_ranges_and_remaps() {
         let directory = Directory::new();