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/ui-egui/tests/ui_keyframes.rs

@@ -348,3 +348,53 @@ fn graph_editor_buttons_that_do_not_fit_move_to_the_more_menu() {
     click(&mut h, pos2(item[0] + item[2] / 2.0, item[1] + item[3] / 2.0));
     assert_eq!(h.state().ui.timeline.graph_transform_box, !before, "the menu item runs the button");
 }
+
+/// #355, #491: with three layers selected the Properties panel heads "(3) Selected Objects"; its
+/// stopwatch animates Position on all three (one undo step) and scrubbing X moves all three by
+/// the same amount.
+#[test]
+fn properties_panel_edits_every_selected_layer() {
+    let mut s = Session::default();
+    s.execute("comp.new", json!({"name": "Three", "width": 320, "height": 180, "frameRate": 30, "duration": 4})).unwrap();
+    let mut ids = vec![];
+    for (i, c) in ["#e04020", "#20e040", "#2040e0"].iter().enumerate() {
+        let l = s.execute("layer.newSolid", json!({"color": c, "width": 40, "height": 40})).unwrap()["layer"].as_u64().unwrap();
+        s.execute("prop.set", json!({"layer": l, "path": "transform/position", "value": [60.0 + 80.0 * i as f64, 90.0]})).unwrap();
+        ids.push(l);
+    }
+    s.execute("layer.select", json!({"layers": ids})).unwrap();
+    let first = s.state.selected_layers[0];
+    let uid = s.active_comp().unwrap().layer(first).unwrap().props.prop("transform/position").unwrap().uid;
+    let mut h = Harness::builder().with_size(egui::vec2(1600.0, 1000.0)).build_eframe(|_| EffectcraftApp::new(s));
+    h.state_mut().show_panel(effectcraft_ui_egui::dock::PanelKind::Properties);
+    h.run_steps(4);
+    assert_eq!(h.state().auto.find("properties.selectedObjects").map(|e| e.label.clone()), Some("(3) Selected Objects".into()));
+    let pos = |h: &Harness<'_, EffectcraftApp>, l: u64| {
+        h.state().session.active_comp().unwrap().layer(LayerId(l)).unwrap().props.prop("transform/position").unwrap().clone()
+    };
+    let at = |h: &Harness<'_, EffectcraftApp>, id: &str| {
+        let e = h.state().auto.find(id).unwrap_or_else(|| panic!("no {id}"));
+        pos2(e.rect[0] + e.rect[2] / 2.0, e.rect[1] + e.rect[3] / 2.0)
+    };
+    let sw = at(&h, &format!("properties.prop.{uid}.stopwatch"));
+    h.input_mut().events.push(Event::PointerMoved(sw));
+    h.step();
+    h.input_mut().events.push(Event::PointerButton { pos: sw, button: egui::PointerButton::Primary, pressed: true, modifiers: Default::default() });
+    h.step();
+    h.input_mut().events.push(Event::PointerButton { pos: sw, button: egui::PointerButton::Primary, pressed: false, modifiers: Default::default() });
+    h.run_steps(3);
+    assert!(ids.iter().all(|l| pos(&h, *l).is_animated()), "every selected layer's stopwatch");
+    h.state_mut().session.execute("edit.undo", json!({})).unwrap();
+    h.run_steps(2);
+    assert!(ids.iter().all(|l| !pos(&h, *l).is_animated()), "one undo step");
+    // Scrub X: each layer moves by the same amount.
+    let before: Vec<f64> = ids.iter().map(|l| pos(&h, *l).value.components()[0]).collect();
+    let x = at(&h, &format!("properties.prop.{uid}.value.0"));
+    drag(&mut h, x, x + egui::vec2(30.0, 0.0), Default::default());
+    let after: Vec<f64> = ids.iter().map(|l| pos(&h, *l).value.components()[0]).collect();
+    let d = after[0] - before[0];
+    assert!(d.abs() > 1.0, "the scrub moved X: {before:?} → {after:?}");
+    for (a, b) in after.iter().zip(&before) {
+        assert!((a - b - d).abs() < 1e-6, "same change on every layer: {before:?} → {after:?}");
+    }
+}