Media: stop a clip across slides when the next slide reuses its id (#994)

A clip set to play across slides kept its voice even when the next slide
holds a different clip under the same shape id, so that slide's own clip
never started. Keep the voice only when the next slide has no clip under
that id or the same media.

Co-Authored-By: Claude <noreply@anthropic.com>

crates/ui-egui/src/media.rs

@@ -337,14 +337,27 @@ impl MediaHost {
 /// The clip of media shape `id`. Ids are only unique per slide (the PPTX import numbers each slide
 /// afresh), so the `near` slide is searched first, then the rest of the deck.
 pub fn find_clip(doc: &Presentation, id: ShapeId, near: Option<usize>) -> Option<MediaClip> {
-    fn walk(shapes: &[deckcraft_model::Shape], id: ShapeId) -> Option<MediaClip> {
-        shapes.iter().find_map(|s| match &s.kind {
-            ShapeKind::Media(m) if s.id == id => Some(m.clone()),
-            ShapeKind::Group { children, .. } => walk(children, id),
-            _ => None,
-        })
-    }
-    near.and_then(|i| doc.slides.get(i)).and_then(|s| walk(&s.shapes, id)).or_else(|| doc.slides.iter().find_map(|s| walk(&s.shapes, id)))
+    near.and_then(|i| clip_on(doc, i, id)).or_else(|| doc.slides.iter().find_map(|s| walk_clip(&s.shapes, id)))
+}
+
+/// The clip `id` on slide `index` only (inside groups too).
+pub fn clip_on(doc: &Presentation, index: usize, id: ShapeId) -> Option<MediaClip> {
+    doc.slides.get(index).and_then(|s| walk_clip(&s.shapes, id))
+}
+
+fn walk_clip(shapes: &[deckcraft_model::Shape], id: ShapeId) -> Option<MediaClip> {
+    shapes.iter().find_map(|s| match &s.kind {
+        ShapeKind::Media(m) if s.id == id => Some(m.clone()),
+        ShapeKind::Group { children, .. } => walk_clip(children, id),
+        _ => None,
+    })
+}
+
+/// Whether the voice `id`, playing on slide `from`, keeps playing on slide `to`: its clip plays
+/// across slides and `to` doesn't hold another clip under the same id.
+pub fn carries_over(doc: &Presentation, id: ShapeId, from: Option<usize>, to: usize) -> bool {
+    let Some(clip) = find_clip(doc, id, from).filter(|c| c.play_across_slides) else { return false };
+    clip_on(doc, to, id).is_none_or(|c| c.media == clip.media)
 }
 
 /// Media shapes directly on slide `index`: (id, box in slide points, clip).
@@ -633,4 +646,27 @@ mod tests {
         assert_eq!(find_clip(&doc, id, None).map(|c| c.media), Some(first.media), "without a hint: first match");
         assert_eq!(find_clip(&doc, id, Some(9)).map(|c| c.media), Some(first.media), "a slide that doesn't exist");
     }
+
+    #[test]
+    fn a_clip_across_slides_stops_where_the_next_slide_reuses_its_id() {
+        let (mut doc, id, first) = deck(4000, wav_level(0.05, 0.5));
+        if let ShapeKind::Media(m) = &mut Arc::make_mut(&mut doc.slides[0]).shapes.last_mut().expect("shape").kind {
+            m.play_across_slides = true;
+        }
+        let mut plain = (*doc.slides[0]).clone();
+        plain.shapes.clear();
+        doc.slides.push(Arc::new(plain));
+        assert!(carries_over(&doc, id, Some(0), 1), "a slide without that id");
+        let mut same = (*doc.slides[0]).clone();
+        if let ShapeKind::Media(m) = &mut same.shapes.last_mut().expect("shape").kind {
+            m.play_across_slides = false;
+        }
+        doc.slides.push(Arc::new(same));
+        assert!(carries_over(&doc, id, Some(0), 2), "the same media under that id");
+        let other = doc.add_media("two.wav", "audio/wav", wav_level(0.05, 0.25)).expect("media");
+        let mut reused = (*doc.slides[0]).clone();
+        reused.shapes.last_mut().expect("shape").kind = ShapeKind::Media(MediaClip { media: other, volume: 1.0, autoplay: true, ..first.clone() });
+        doc.slides.push(Arc::new(reused));
+        assert!(!carries_over(&doc, id, Some(0), 3), "another clip under that id");
+    }
 }

crates/ui-egui/src/show.rs

@@ -1077,11 +1077,11 @@ fn show_media(app: &mut SlideApp, show: &mut Show, doc_uid: u64, doc: &Presentat
     // [Stop Previous Sound] ends a transition sound still playing.
     let stop_previous = trans.is_some_and(|t| t.stop_sound);
     let keep: Vec<ShapeId> = match cur {
-        Some(_) => app
+        Some(i) => app
             .media
             .ids()
             .into_iter()
-            .filter(|id| crate::media::find_clip(doc, *id, previous).is_some_and(|c| c.play_across_slides))
+            .filter(|id| crate::media::carries_over(doc, *id, previous, i))
             // A transition sound plays to its end, as in PowerPoint, unless the new slide has its own.
             .chain((sound.is_none() && !stop_previous).then_some(TRANSITION_SOUND).filter(|id| app.media.is_playing(*id)))
             .collect(),