Timeline: scrolling the video tracks up leaves room to drop a new track (fixes #1522)

crates/ui-egui/src/panels/timeline.rs

@@ -233,6 +233,19 @@ fn new_track_at(seq: &Sequence, layout: &Layout, y: f32, vh: f32, ah: f32) -> Op
     }
 }
 
+/// The top of each of `n` video rows `vh` high, V1 first: V1 sits at the bottom of `area` and the
+/// tracks stack upward; `v_scroll` (clamped here) shifts them up. Also gives the largest `v_scroll`.
+/// The scroll range includes one empty track's height above the top track: without it, tracks that
+/// fill the area leave no space where a drop adds a new one (#1522).
+fn video_row_tops(area: Rect, n: usize, vh: f32, v_scroll: &mut f32) -> (Vec<f32>, f32) {
+    let total = (n + 1) as f32 * vh;
+    let v_off = (area.height() - total).max(0.0);
+    let max_vs = (total - area.height()).max(0.0);
+    *v_scroll = v_scroll.clamp(0.0, max_vs);
+    let tops = (0..n).map(|i| area.min.y + v_off + (n - i) as f32 * vh - (max_vs - *v_scroll)).collect();
+    (tops, max_vs)
+}
+
 /// How far the Timeline reaches, in seconds: ten minutes past the end of the sequence. Premiere
 /// Pro's Timeline does the same (on a 6 s sequence its view stops with 00:10:06 at the right
 /// edge). The view cannot be scrolled past it and the scroll bar spans it.
@@ -412,14 +425,9 @@ pub fn show(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
     let vh = app.ui.timeline.video_track_h;
     let ah = app.ui.timeline.audio_track_h;
     let mut rows = Vec::new();
-    // Video: V1 at the bottom of the video area, stacking upward; v_scroll shifts up.
     let nv = seq.video_tracks.len();
-    let video_total = nv as f32 * vh;
-    let v_off = (video_area.height() - video_total).max(0.0);
-    let max_vs = (video_total - video_area.height()).max(0.0);
-    app.ui.timeline.v_scroll = app.ui.timeline.v_scroll.clamp(0.0, max_vs);
-    for (i, tr) in seq.video_tracks.iter().enumerate() {
-        let top = video_area.min.y + v_off + (nv - 1 - i) as f32 * vh - (max_vs - app.ui.timeline.v_scroll);
+    let (video_tops, max_vs) = video_row_tops(video_area, nv, vh, &mut app.ui.timeline.v_scroll);
+    for ((i, tr), top) in seq.video_tracks.iter().enumerate().zip(video_tops) {
         let rect = Rect::from_min_max(pos2(content.min.x, top), pos2(content.max.x, top + vh));
         rows.push(Row { track: tr.id, kind: TrackKind::Video, index: i, rect, lane: false });
     }
@@ -3201,4 +3209,42 @@ mod new_track_tests {
             assert_eq!(at(y), None, "y = {y}");
         }
     }
+
+    /// #1522: when the video tracks fill the video area, scrolling all the way up still leaves a
+    /// track's height of empty space above the top one, so a drop there can add a fourth track.
+    #[test]
+    fn a_full_video_area_scrolls_to_space_for_a_new_track() {
+        let mut p = Project::new("t");
+        let id = p.new_sequence("s", SequenceSettings::default(), 3, 1, None);
+        let seq = p.sequence(id).cloned().expect("sequence");
+        // video area 0..150 holds V1..V3 exactly; scrolled up as far as it goes
+        let area = Rect::from_min_max(pos2(0.0, 0.0), pos2(800.0, 150.0));
+        let mut v_scroll = f32::MAX;
+        let (tops, _) = video_row_tops(area, 3, 50.0, &mut v_scroll);
+        let rows = seq
+            .video_tracks
+            .iter()
+            .zip(&tops)
+            .enumerate()
+            .map(|(index, (t, &y0))| Row {
+                track: t.id,
+                kind: TrackKind::Video,
+                index,
+                rect: Rect::from_min_max(pos2(0.0, y0), pos2(800.0, y0 + 50.0)),
+                lane: false,
+            })
+            .collect();
+        let layout = Layout {
+            content: Rect::from_min_max(pos2(0.0, 0.0), pos2(800.0, 500.0)),
+            ruler: Rect::from_min_max(pos2(0.0, -20.0), pos2(800.0, 0.0)),
+            rows,
+            pps: 100.0,
+            scroll: 0.0,
+            split_y: 150.0,
+        };
+        assert_eq!(new_track_at(&seq, &layout, 10.0, 50.0, 50.0), Some((TrackKind::Video, Rect::from_min_max(pos2(0.0, 0.0), pos2(800.0, 50.0)))));
+        // scrolled to the bottom, V1..V3 fill the area as before
+        let mut v_scroll = 0.0;
+        assert_eq!(video_row_tops(area, 3, 50.0, &mut v_scroll).0, vec![100.0, 50.0, 0.0]);
+    }
 }