Layers: with several layers selected, value edits, stopwatches and viewer Scale / Rotate change them all (#355, #491)

With several layers selected, After Effects applies a property edit to the same property of
every selected layer; we changed only the layer whose row was edited (the Properties panel
showed only the first selected layer), so a stopwatch switched on one layer and later moves
of the others had nothing to keyframe (#355), and scaling or rotating a selection changed
one layer (#491).

- Engine: `prop.set`, `prop.toggleAnimation` and `prop.toggleKey` take `selected: true`:
  the change also goes to the same property (by match path, `transform/scale`,
  `effects/<effect>#n/<param>`) of every other selected, unlocked layer whose value has the
  same kind, in one undo step (a scrub keeps folding into it through its `merge` key). With
  `offset: true` (a scrub) each layer changes by the same amount from its own value; a typed
  value sets them all. Position with Separate Dimensions is handled per layer.
- The Timeline, Properties and Effect Controls panels pass `selected` (and `offset` while a
  value is dragged). The Properties panel heads "(N) Selected Objects" (zh-CN / zh-TW / ja /
  uk rows).
- Viewer: a bounding-box handle drag scales every selected layer by the same ratio about its
  own anchor point; the Rotation tool turns them all by the same angle (one undo step).

Tests: engine test (stopwatch, scrub offset, typed value, keyframe button, one undo step,
unselected layers untouched); kittests: the Properties panel with three layers selected
(header, stopwatch on all three undone in one step, a scrub moving all three by the same
amount) and a handle drag scaling two selected layers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QiAJeFttZp83Fh7YuzAR5f

crates/engine/src/tests_timeline.rs

@@ -831,3 +831,51 @@ fn toggle_transform_key_covers_selected_layers_separated_position_and_3d_rotatio
     }
     assert!(s.execute("keys.toggleTransform", json!({"prop": "skew"})).is_err());
 }
+
+/// #355, #491: with several layers selected, the stopwatch, the keyframe button and value edits
+/// (`selected: true`, as the panels send them) apply to every selected layer in one undo step; a
+/// scrub (`offset`) moves each layer by the same amount, a typed value sets them all.
+#[test]
+fn property_edits_apply_to_every_selected_layer() {
+    let (mut s, a) = setup();
+    let b = s.execute("layer.newSolid", json!({"color": "#ff0000", "width": 50, "height": 50})).unwrap()["layer"].as_u64().unwrap();
+    let c = s.execute("layer.newSolid", json!({"color": "#00ff00", "width": 50, "height": 50})).unwrap()["layer"].as_u64().unwrap();
+    s.execute("prop.set", json!({"layer": b, "path": "transform/position", "value": [10, 20]})).unwrap();
+    s.execute("layer.select", json!({"layers": [a, b]})).unwrap();
+    let pos = |s: &Session, l: u64| prop(s, l, "transform/position").value_at(effectcraft_time::Tick::ZERO).components();
+    let uid = prop(&s, a, "transform/position").uid;
+
+    // Stopwatch on all selected layers, one undo step; the unselected layer is untouched.
+    let undo = s.history.undo.len();
+    s.execute("prop.toggleAnimation", json!({"layer": a, "prop": uid, "selected": true})).unwrap();
+    assert!(prop(&s, a, "transform/position").is_animated() && prop(&s, b, "transform/position").is_animated());
+    assert!(!prop(&s, c, "transform/position").is_animated());
+    assert_eq!(s.history.undo.len(), undo + 1);
+    s.execute("edit.undo", json!({})).unwrap();
+    assert!(!prop(&s, a, "transform/position").is_animated() && !prop(&s, b, "transform/position").is_animated());
+
+    // A scrub moves both by the same change (merged into one step while it continues).
+    let (pa, pb) = (pos(&s, a), pos(&s, b));
+    for k in 1..=3 {
+        let v = [pa[0] + 5.0 * k as f64, pa[1]];
+        s.execute("prop.set", json!({"layer": a, "prop": uid, "value": v, "selected": true, "offset": true, "merge": "scrub-1"})).unwrap();
+    }
+    assert_eq!(pos(&s, a)[..2], [pa[0] + 15.0, pa[1]]);
+    assert_eq!(pos(&s, b)[..2], [pb[0] + 15.0, pb[1]]);
+    assert_eq!(s.history.undo.len(), undo + 1, "one undo step for the scrub");
+    // A typed value sets them all.
+    s.execute("prop.set", json!({"layer": a, "path": "transform/opacity", "value": 40, "selected": true})).unwrap();
+    assert_eq!(prop(&s, b, "transform/opacity").value.as_f64(), 40.0);
+    assert_eq!(prop(&s, c, "transform/opacity").value.as_f64(), 100.0);
+    s.execute("edit.undo", json!({})).unwrap();
+    assert_eq!(prop(&s, a, "transform/opacity").value.as_f64(), 100.0);
+    assert_eq!(prop(&s, b, "transform/opacity").value.as_f64(), 100.0);
+
+    // The keyframe button adds a key on each selected layer.
+    s.execute("prop.toggleKey", json!({"layer": a, "path": "transform/scale", "selected": true})).unwrap();
+    assert_eq!(prop(&s, a, "transform/scale").keys.len(), 1);
+    assert_eq!(prop(&s, b, "transform/scale").keys.len(), 1);
+    // Without `selected` only the given layer changes.
+    s.execute("prop.set", json!({"layer": a, "path": "transform/rotation", "value": 30})).unwrap();
+    assert_eq!(prop(&s, b, "transform/rotation").value.as_f64(), 0.0);
+}