Merge pull request #1258 from gyroninja/fix/transitions-follow-clip-drag

Fix clip drags leaving transitions behind

crates/edit/src/lib.rs

@@ -559,7 +559,8 @@ pub fn copy_items(seq: &mut Sequence, moves: &[(ClipId, TrackId, Tick)], insert_
         it.start = (*start).max(Tick::ZERO);
         placed.push((*dest, it));
     }
-    // worked out on the original ids, then moved over to the copies
+    // worked out on the original ids, then moved over to the copies (only transitions whose every
+    // clip is copied travel; one whose partner stays with the originals is not duplicated)
     let carried = carried_transitions(&work, &placed);
     let mut ids = HashMap::new();
     let mut links = HashMap::new();
@@ -596,9 +597,10 @@ pub fn copy_items(seq: &mut Sequence, moves: &[(ClipId, TrackId, Tick)], insert_
     Ok(out)
 }
 
-/// The transitions that travel with a move (#227, #374): those whose clips all move, onto one
-/// track and by one offset (a fade at a moved clip's edge, or the cut between two clips that move
-/// together), shifted to where those clips land.
+/// The transitions that travel with a move (#227, #374): only those whose every clip moves, onto
+/// one track and by one offset (a fade at a moved clip's edge, or the cut between two clips that
+/// move together), shifted to where those clips land. A transition whose partner stays behind is
+/// dropped, as in Premiere (the drag preview leaves it where it is until the release).
 fn carried_transitions(seq: &Sequence, placed: &[(TrackId, TrackItem)]) -> Vec<(TrackId, Transition)> {
     let landing = |c: ClipId| {
         let (dest, it) = placed.iter().find(|(_, it)| it.id == c)?;
@@ -607,9 +609,14 @@ fn carried_transitions(seq: &Sequence, placed: &[(TrackId, TrackItem)]) -> Vec<(
     };
     let mut carried = Vec::new();
     for tr in seq.all_tracks().filter(|t| !t.locked).flat_map(|t| t.transitions.iter()) {
-        let lands: Option<Vec<(TrackId, Tick)>> = [tr.from, tr.to].into_iter().flatten().map(landing).collect();
-        let Some((&(dest, delta), rest)) = lands.as_deref().and_then(<[_]>::split_first) else { continue };
-        if rest.iter().all(|l| *l == (dest, delta)) {
+        let endpoints: Vec<ClipId> = [tr.from, tr.to].into_iter().flatten().collect();
+        let lands: Vec<(TrackId, Tick)> = endpoints.iter().filter_map(|c| landing(*c)).collect();
+        // every clip it sits on must move, onto one track by one offset, or the transition is
+        // dropped with the delete below (a partner that stayed behind ends the move)
+        if lands.len() == endpoints.len()
+            && let Some(&(dest, delta)) = lands.first()
+            && lands.iter().skip(1).all(|l| *l == (dest, delta))
+        {
             carried.push((dest, Transition { start: tr.start + delta, ..tr.clone() }));
         }
     }

crates/edit/src/tests.rs

@@ -554,7 +554,7 @@ fn transitions_travel_with_moved_clips() {
     // a cut whose two clips move together keeps its transition, in an insert move as well
     move_items(&mut fx.seq, &[(b, v1, f(60)), (c, v1, f(70))], true, &mut Fx::ctx(&mut n)).unwrap();
     assert_eq!(at(&fx, v1), vec![(2, 68), (3, 76)]);
-    // a crossfade whose partner stays behind is dropped; the other clip's fade stays with it
+    // a crossfade whose partner stays behind is dropped; the moving clip's own fade stays with it
     move_items(&mut fx.seq, &[(c, v1, f(90))], false, &mut Fx::ctx(&mut n)).unwrap();
     assert_eq!(at(&fx, v1), vec![(3, 96)]);
     fx.seq.check().unwrap();

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

@@ -566,22 +566,39 @@ pub fn show(app: &mut FilmcraftApp, ui: &mut egui::Ui, rect: Rect) {
         }
         // selected edit points (trim mode): red brackets for ripple, yellow for roll / regular trim
         super::trim_monitor::paint_edit_points(app, &p, seq, tr, r, &layout);
-        // transitions (one being dragged where the drag puts it; the selected ones outlined)
+        // transitions (one being dragged where the drag puts it; the selected ones outlined). A
+        // clip drag also shows where the drop will carry its transitions (those whose clips all
+        // move to one track by one offset): a plain move draws them on their destination track
+        // right away, a copy adds a ghost beside the copies while the originals stay at home.
         for trn in &tr.transitions {
+            let carried = move_transition_preview(app, seq, trn);
+            // the transition's home span (a transition dragged by an end still takes over)
             let (start, end) = match &app.tl.drag {
                 Some(Drag::Transition { id, start, duration, .. }) if *id == trn.id => (*start, *start + *duration),
                 _ => (trn.start, trn.end()),
             };
-            let x0 = layout.x_of(start);
-            let x1 = layout.x_of(end);
-            let tr_rect = Rect::from_min_max(pos2(x0, r.rect.min.y + 17.0), pos2(x1, r.rect.max.y - 1.0));
-            let selected = app.session.state.transition_selection.contains(&trn.id);
-            draw_transition(&p, tr_rect, trn, selected, &t);
-            app.auto.add(&format!("timeline.transition.{}", trn.id.0), tr_rect, &trn.effect.effect);
-            // its ends, for dragging the duration (the same zones as the hit test)
-            let zone = super::timeline_hit::EDGE_PX.min(tr_rect.width() / 3.0);
-            app.auto.add(&format!("timeline.transition.{}.in", trn.id.0), Rect::from_min_max(tr_rect.min, pos2(x0 + zone, tr_rect.max.y)), "start");
-            app.auto.add(&format!("timeline.transition.{}.out", trn.id.0), Rect::from_min_max(pos2(x1 - zone, tr_rect.min.y), tr_rect.max), "end");
+            // (start, end, is the copy's ghost)
+            let show: Vec<(Tick, Tick, bool)> = match (carried, copying) {
+                (Some((s, e, dest)), false) if dest == r.track => vec![(s, e, false)],
+                (Some((s, e, dest)), true) if dest == r.track => vec![(start, end, false), (s, e, true)],
+                (Some(_), false) => Vec::new(), // drawn on its destination row
+                _ => vec![(start, end, false)], // at home: not carried, or a copy's original
+            };
+            for (start, end, ghost) in show {
+                let x0 = layout.x_of(start);
+                let x1 = layout.x_of(end);
+                let tr_rect = Rect::from_min_max(pos2(x0, r.rect.min.y + 17.0), pos2(x1, r.rect.max.y - 1.0));
+                let selected = app.session.state.transition_selection.contains(&trn.id);
+                draw_transition(&p, tr_rect, trn, ghost || selected, &t);
+                let suffix = if ghost { ".copy" } else { "" };
+                app.auto.add(&format!("timeline.transition.{}{}", trn.id.0, suffix), tr_rect, &trn.effect.effect);
+                // its ends, for dragging the duration (the same zones as the hit test)
+                let zone = super::timeline_hit::EDGE_PX.min(tr_rect.width() / 3.0);
+                if !ghost {
+                    app.auto.add(&format!("timeline.transition.{}.in", trn.id.0), Rect::from_min_max(tr_rect.min, pos2(x0 + zone, tr_rect.max.y)), "start");
+                    app.auto.add(&format!("timeline.transition.{}.out", trn.id.0), Rect::from_min_max(pos2(x1 - zone, tr_rect.min.y), tr_rect.max), "end");
+                }
+            }
         }
     }
 
@@ -1858,6 +1875,38 @@ fn preview_drag(app: &FilmcraftApp, seq: &Sequence, out: &mut HashMap<ClipId, (T
     }
 }
 
+/// The transition a clip drag will leave behind, by the same rule `edit::move_items` applies:
+/// the transition travels when every clip it sits on moves — to one track, by the same (clamped)
+/// offset — and is dropped when only some of its clips move (a partner that stays behind ends the
+/// move, as in Premiere), shifted by that offset. The preview draws carried ones where they land,
+/// so a dragged clip's transition follows it instead of lagging behind until the drop, and a cut
+/// transition does not ride along on a clip whose partner stays. Returns the transition's
+/// `(start, end, destination track)` when the drag carries it.
+fn move_transition_preview(app: &FilmcraftApp, seq: &Sequence, trn: &filmcraft_project::Transition) -> Option<(Tick, Tick, TrackId)> {
+    let Some(Drag::Move { clips, offset, track_delta, .. }) = &app.tl.drag else { return None };
+    let endpoints: Vec<ClipId> = [trn.from, trn.to].into_iter().flatten().collect();
+    // where each clip it sits on that moves lands (the same track shift as the clip preview), and
+    // by how much: clamped like the drop clamps each clip's start
+    let lands: Vec<(TrackId, Tick)> = endpoints
+        .iter()
+        .filter_map(|c| {
+            if !clips.contains(c) {
+                return None;
+            }
+            let (tid, it) = seq.find_item(*c)?;
+            let dest = shift_track(seq, tid, *track_delta)?;
+            Some((dest, (it.start + *offset).max(Tick::ZERO) - it.start))
+        })
+        .collect();
+    // every clip it sits on must move to one track by one offset; one staying behind keeps the
+    // transition where it is (and the drop does not carry it either)
+    if lands.len() != endpoints.len() {
+        return None;
+    }
+    let &(dest, delta) = lands.first()?;
+    lands.iter().skip(1).all(|l| *l == (dest, delta)).then(|| (trn.start + delta, trn.end() + delta, dest))
+}
+
 /// The clips a marquee rectangle touches, on the tracks it crosses.
 fn marquee_clips(seq: &Sequence, layout: &Layout, r: Rect) -> Vec<ClipId> {
     let (a, b) = (layout.tick_at(r.min.x), layout.tick_at(r.max.x));

crates/ui-egui/tests/timeline_transition_move_ui.rs

@@ -0,0 +1,238 @@
+//! Dragging clips previews the transitions the drop will carry, exactly as the drop re-adds them
+//! (`edit::move_items`): a fade follows its clip, a cut transition travels with its clips only
+//! when every one of them moves — dragging a clip out from under one side of a cut leaves the
+//! transition where it was until the release deletes it (Premiere) — an Option-drag adds a
+//! traveling copy while the original stays.
+
+use egui::{Event, Modifiers, PointerButton, Pos2, Rect, pos2, vec2};
+use egui_kittest::Harness;
+use filmcraft_engine::Session;
+use filmcraft_ui_egui::FilmcraftApp;
+use serde_json::json;
+
+fn step(h: &mut Harness<'static, FilmcraftApp>) {
+    let ctx = h.ctx.clone();
+    let mut raw = std::mem::take(h.input_mut());
+    eframe::App::raw_input_hook(h.state_mut(), &ctx, &mut raw);
+    *h.input_mut() = raw;
+    ctx.request_repaint();
+    h.step();
+}
+
+fn pointer(h: &mut Harness<'static, FilmcraftApp>, pos: Pos2, pressed: Option<bool>) {
+    h.input_mut().events.push(Event::PointerMoved(pos));
+    if let Some(pressed) = pressed {
+        h.input_mut().events.push(Event::PointerButton { pos, button: PointerButton::Primary, pressed, modifiers: Modifiers::NONE });
+    }
+    step(h);
+}
+
+fn rect(h: &Harness<'static, FilmcraftApp>, id: &str) -> Rect {
+    let [x, y, w, height] = h.state().auto.elements.iter().find(|e| e.id == id).unwrap_or_else(|| panic!("missing {id}")).rect;
+    Rect::from_min_size(pos2(x, y), vec2(w, height))
+}
+
+fn build(s: Session) -> Harness<'static, FilmcraftApp> {
+    let app = FilmcraftApp::new(s);
+    let mut builder = Harness::builder().with_size(vec2(1600.0, 980.0)).with_max_steps(10_000);
+    if std::env::var_os("FILMCRAFT_UI_SNAPSHOT_DIR").is_some() {
+        builder = builder.wgpu();
+    }
+    let mut h = builder.build_eframe(move |_cc| app);
+    for _ in 0..4 {
+        step(&mut h);
+    }
+    // Opening the new sequence restores its own view; set the test zoom after that initial fit.
+    h.state_mut().ui.timeline.pps = 60.0;
+    h.state_mut().ui.timeline.target_pps = 60.0;
+    h.state_mut().ui.timeline.fit_pending = false;
+    step(&mut h);
+    h
+}
+
+/// A sequence with a cross dissolve: between two clips at 0..48f and 48..96f, or a fade-in on the
+/// single clip at 0..48f when `single`.
+fn harness(single: bool) -> Harness<'static, FilmcraftApp> {
+    let mut s = Session::default();
+    s.execute("file.openDemoProject", json!({})).unwrap();
+    s.execute("file.newSequence", json!({"name":"Transition move","fps":24})).unwrap();
+    let rate = s.sequence_rate();
+    for (at, frames) in if single { vec![(0, 48)] } else { vec![(0, 48), (48, 48)] } {
+        s.execute("timeline.place", json!({"item":5,"time":rate.tick_of(at).0,"duration":rate.tick_of(frames).0})).unwrap();
+    }
+    let seq = s.active_sequence().unwrap().clone();
+    let clip = seq.video_tracks[0].items[if single { 0 } else { 1 }].id;
+    s.execute("effects.apply", json!({"effect": "cross_dissolve", "clip": clip.0, "edge": "in"})).unwrap();
+    build(s)
+}
+
+/// XYZ with a cut transition on each cut: X↔Y and Y↔Z.
+fn harness_xyz() -> Harness<'static, FilmcraftApp> {
+    let mut s = Session::default();
+    s.execute("file.openDemoProject", json!({})).unwrap();
+    s.execute("file.newSequence", json!({"name":"Transition move","fps":24})).unwrap();
+    let rate = s.sequence_rate();
+    for (at, frames) in [(0, 48), (48, 48), (96, 48)] {
+        s.execute("timeline.place", json!({"item":5,"time":rate.tick_of(at).0,"duration":rate.tick_of(frames).0})).unwrap();
+    }
+    let seq = s.active_sequence().unwrap().clone();
+    let (y, z) = (seq.video_tracks[0].items[1].id, seq.video_tracks[0].items[2].id);
+    for clip in [y, z] {
+        s.execute("effects.apply", json!({"effect": "cross_dissolve", "clip": clip.0, "edge": "in"})).unwrap();
+    }
+    build(s)
+}
+
+/// A drag preview that keeps a transition's offset to its clip (it follows, not lags), and a drop
+/// that re-adds the transition shifted by exactly the move.
+#[test]
+fn dragging_a_clip_carries_its_fade_in_the_preview_and_on_the_drop() {
+    let mut h = harness(true);
+    let q0 = h.state().session.active_sequence().unwrap().clone();
+    let (a0, tr0) = (q0.video_tracks[0].items[0].clone(), q0.video_tracks[0].transitions[0].clone());
+    let a_rect = rect(&h, &format!("timeline.clip.{}", a0.id.0));
+    let tr_rect = rect(&h, &format!("timeline.transition.{}", tr0.id.0));
+    assert_eq!(tr0.to, Some(a0.id), "the fade-in sits on the dragged clip");
+    let gap_before = tr_rect.min.x - a_rect.min.x;
+    let y = a_rect.center().y;
+    // press clear of the fade band (it covers the left 75 px of the clip) and of the edge zones
+    let x0 = a_rect.left() + 110.0;
+    pointer(&mut h, pos2(x0, y), Some(true));
+    for i in 1..=6 {
+        pointer(&mut h, pos2(x0 + 20.0 * i as f32, y), None);
+    }
+    // mid-drag: the fade follows the clip's ghost, keeping its offset to the clip edge
+    let gap_mid = rect(&h, &format!("timeline.transition.{}", tr0.id.0)).min.x - rect(&h, &format!("timeline.clip.{}", a0.id.0)).min.x;
+    assert!((gap_mid - gap_before).abs() < 1.0, "fade lagged behind: gap {gap_before} -> {gap_mid}");
+    pointer(&mut h, pos2(x0 + 140.0, y), Some(false));
+    let q1 = h.state().session.active_sequence().unwrap().clone();
+    let (a1, tr1) = (q1.video_tracks[0].items[0].clone(), q1.video_tracks[0].transitions[0].clone());
+    assert_eq!(a1.start.0 - a0.start.0, tr1.start.0 - tr0.start.0, "the committed transition moved with the clip");
+    assert_eq!((tr1.from, tr1.duration), (tr0.from, tr0.duration));
+}
+
+/// Both clips of a cut transition move together: the transition travels with them in the preview
+/// and is re-added at the shared delta.
+#[test]
+fn dragging_both_clips_of_a_cut_carries_the_transition() {
+    let mut h = harness(false);
+    let q0 = h.state().session.active_sequence().unwrap().clone();
+    let (a0, b0, tr0) = (q0.video_tracks[0].items[0].clone(), q0.video_tracks[0].items[1].clone(), q0.video_tracks[0].transitions[0].clone());
+    h.state_mut().session.execute("timeline.select", json!({"clips": [a0.id.0, b0.id.0]})).unwrap();
+    step(&mut h);
+    let a_rect = rect(&h, &format!("timeline.clip.{}", a0.id.0));
+    let tr_rect = rect(&h, &format!("timeline.transition.{}", tr0.id.0));
+    let gap_before = tr_rect.min.x - a_rect.min.x;
+    let y = a_rect.center().y;
+    pointer(&mut h, a_rect.center(), Some(true));
+    for i in 1..=6 {
+        pointer(&mut h, pos2(a_rect.center().x + 20.0 * i as f32, y), None);
+    }
+    let gap_mid = rect(&h, &format!("timeline.transition.{}", tr0.id.0)).min.x - rect(&h, &format!("timeline.clip.{}", a0.id.0)).min.x;
+    assert!((gap_mid - gap_before).abs() < 1.0, "transition lagged: gap {gap_before} -> {gap_mid}");
+    pointer(&mut h, pos2(a_rect.center().x + 140.0, y), Some(false));
+    let q1 = h.state().session.active_sequence().unwrap().clone();
+    let (a1, b1, tr1) = (q1.video_tracks[0].items[0].clone(), q1.video_tracks[0].items[1].clone(), q1.video_tracks[0].transitions[0].clone());
+    assert_eq!(a1.start.0 - a0.start.0, tr1.start.0 - tr0.start.0);
+    assert_eq!(b1.start.0 - b0.start.0, tr1.start.0 - tr0.start.0, "the transition moved with both clips");
+}
+
+/// XYZ with cut transitions on both cuts: dragging YZ leaves the X↔Y transition where it is
+/// while dragging (a preview of the drop that deletes it, as in Premiere) and the Y↔Z one
+/// travels with the pair.
+#[test]
+fn dragging_two_clips_drops_the_transition_on_the_cut_that_stayed() {
+    let mut h = harness_xyz();
+    let q0 = h.state().session.active_sequence().unwrap().clone();
+    let (x0, y0, z0) = (q0.video_tracks[0].items[0].clone(), q0.video_tracks[0].items[1].clone(), q0.video_tracks[0].items[2].clone());
+    let (xy0, yz0) = (q0.video_tracks[0].transitions[0].clone(), q0.video_tracks[0].transitions[1].clone());
+    h.state_mut().session.execute("timeline.select", json!({"clips": [y0.id.0, z0.id.0]})).unwrap();
+    step(&mut h);
+    let y_rect = rect(&h, &format!("timeline.clip.{}", y0.id.0));
+    let xy_rect = rect(&h, &format!("timeline.transition.{}", xy0.id.0));
+    let yz_rect = rect(&h, &format!("timeline.transition.{}", yz0.id.0));
+    let gap_before = yz_rect.min.x - y_rect.min.x;
+    let y_center = y_rect.center().y;
+    // press in the middle of Y, clear of both transition bands and the edge zones
+    let x0p = y_rect.left() + 60.0;
+    pointer(&mut h, pos2(x0p, y_center), Some(true));
+    for i in 1..=6 {
+        pointer(&mut h, pos2(x0p + 20.0 * i as f32, y_center), None);
+    }
+    // mid-drag: the X↔Y transition stays on its cut (the drop will remove it), the Y↔Z one
+    // follows the pair
+    assert_eq!(rect(&h, &format!("timeline.transition.{}", xy0.id.0)), xy_rect, "the X↔Y transition stays on its cut while dragging");
+    let gap_mid = rect(&h, &format!("timeline.transition.{}", yz0.id.0)).min.x - rect(&h, &format!("timeline.clip.{}", y0.id.0)).min.x;
+    assert!((gap_mid - gap_before).abs() < 1.0, "Y↔Z transition lagged: gap {gap_before} -> {gap_mid}");
+    pointer(&mut h, pos2(x0p + 140.0, y_center), Some(false));
+    let q1 = h.state().session.active_sequence().unwrap().clone();
+    let (x1, y1, z1) = (q1.video_tracks[0].items[0].clone(), q1.video_tracks[0].items[1].clone(), q1.video_tracks[0].items[2].clone());
+    assert_eq!(x1.start, x0.start, "X is not part of the drag");
+    assert_eq!(y1.start.0 - y0.start.0, z1.start.0 - z0.start.0, "Y and Z moved together");
+    assert_eq!(q1.video_tracks[0].transitions.len(), 1, "the X↔Y transition was dropped with the move");
+    assert!(!q1.video_tracks[0].transitions.iter().any(|t| t.id == xy0.id), "its id is gone");
+    let yz1 = &q1.video_tracks[0].transitions[0];
+    assert_eq!(yz1.start.0 - yz0.start.0, y1.start.0 - y0.start.0, "the Y↔Z transition traveled with the pair");
+}
+
+/// Option-drag copies the clip and its fade: the original keeps its transition at home, the copy
+/// shows a ghost beside it while dragging and keeps its own transition on the drop.
+#[test]
+fn option_drag_copies_show_a_ghost_transition_with_the_copy() {
+    let mut h = harness(true);
+    let q0 = h.state().session.active_sequence().unwrap().clone();
+    let a0 = q0.video_tracks[0].items[0].clone();
+    let tr0 = q0.video_tracks[0].transitions[0].clone();
+    let a_rect = rect(&h, &format!("timeline.clip.{}", a0.id.0));
+    let tr_rect = rect(&h, &format!("timeline.transition.{}", tr0.id.0));
+    let y = a_rect.center().y;
+    // hold Option for the whole drag, as the real drag does
+    h.input_mut().events.push(egui::Event::ModifiersChanged(Modifiers { alt: true, ..Default::default() }));
+    let x0 = a_rect.left() + 110.0; // clear of the fade band and the edge zones
+    pointer(&mut h, pos2(x0, y), Some(true));
+    for i in 1..=6 {
+        pointer(&mut h, pos2(x0 + 20.0 * i as f32, y), None);
+    }
+    // a ghost transition beside the copy; the original stays exactly where it was
+    let ghost = rect(&h, &format!("timeline.transition.{}.copy", tr0.id.0));
+    assert_eq!(rect(&h, &format!("timeline.transition.{}", tr0.id.0)), tr_rect, "the original transition stays at home");
+    assert!((ghost.min.x - tr_rect.min.x) > 60.0, "ghost {ghost:?} did not follow the copy from {tr_rect:?}");
+    pointer(&mut h, pos2(x0 + 140.0, y), Some(false));
+    h.input_mut().events.push(egui::Event::ModifiersChanged(Modifiers::NONE));
+    step(&mut h);
+    let q1 = h.state().session.active_sequence().unwrap().clone();
+    assert_eq!(q1.video_tracks[0].transitions.len(), 2, "the copy carried its own transition");
+    let (home, copy_tr) = (q1.video_tracks[0].transitions[0].clone(), q1.video_tracks[0].transitions[1].clone());
+    assert_eq!(home.start, tr0.start, "the original's transition stayed");
+    assert_ne!(copy_tr.id, tr0.id, "the copy's transition is a fresh one");
+    let copy = q1.video_tracks[0].items.iter().find(|i| i.id != a0.id).expect("a new clip appeared");
+    assert_eq!(copy_tr.to, Some(copy.id), "the copy's fade sits on the copy");
+}
+
+/// Dragging one side of a cut transition to another track drops it (Premiere); the preview leaves
+/// it where it is, and the drop removes it.
+#[test]
+fn dragging_one_side_to_another_track_drops_the_transition() {
+    let mut h = harness(false);
+    // a clip far to the right on V2 marks that track's row
+    let q = h.state().session.active_sequence().unwrap().clone();
+    let (v2, rate) = (q.video_tracks[1].id, h.state().session.sequence_rate());
+    h.state_mut().session.execute("timeline.place", json!({"item":5,"time":rate.tick_of(200).0,"duration":rate.tick_of(24).0,"track": v2.0})).unwrap();
+    step(&mut h);
+    let q0 = h.state().session.active_sequence().unwrap().clone();
+    let (a0, tr0) = (q0.video_tracks[0].items[0].clone(), q0.video_tracks[0].transitions[0].clone());
+    let a_rect = rect(&h, &format!("timeline.clip.{}", a0.id.0));
+    let v2_rect = rect(&h, &format!("timeline.clip.{}", q0.video_tracks[1].items[0].id.0));
+    let tr_rect = rect(&h, &format!("timeline.transition.{}", tr0.id.0));
+    pointer(&mut h, a_rect.center(), Some(true));
+    for i in 1..=6 {
+        let p = pos2(a_rect.center().x + 20.0 * i as f32, a_rect.center().y + (v2_rect.center().y - a_rect.center().y) * i as f32 / 6.0);
+        // move into the V2 row first (track crossing), so the pointer ends up there
+        pointer(&mut h, p, None);
+    }
+    assert_eq!(rect(&h, &format!("timeline.transition.{}", tr0.id.0)), tr_rect, "the doomed transition stays at home while dragging");
+    pointer(&mut h, pos2(a_rect.center().x + 140.0, v2_rect.center().y), Some(false));
+    let q1 = h.state().session.active_sequence().unwrap().clone();
+    assert!(q1.video_tracks[0].transitions.is_empty(), "the clip moved to V2 and its transition was dropped");
+    assert!(q1.video_tracks.iter().any(|t| t.items.iter().any(|i| i.id == a0.id && t.id == v2)), "the clip landed on V2");
+}