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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01QiAJeFttZp83Fh7YuzAR5f

crates/engine/src/commands/prop.rs

@@ -165,6 +165,145 @@ fn property_reply(p: &Value, layer: &Layer, pr: &Property, time: Tick, value: Va
     }
 }
 
+// ---------------------------------------------------------------- several selected layers
+
+/// With `selected: true` (the Timeline, Properties and Effect Controls panels pass it): the same
+/// property (by match path, `transform/position`, `effects/<effect>#n/<param>`) of every other
+/// selected, unlocked layer of the comp whose value is of the same kind, as (layer, uid). Empty
+/// when `selected` is not set or the edited layer isn't selected. After Effects applies a value
+/// edit, the stopwatch and the keyframe button to all selected layers (#355, #491).
+fn selected_twins(s: &Session, p: &Value, cmd: &str) -> Result<Vec<(LayerId, Uid)>> {
+    if b_p(p, "selected") != Some(true) {
+        return Ok(vec![]);
+    }
+    let (cid, lid, uid) = prop_ref(s, p, cmd)?;
+    let Some(comp) = s.project.comp(cid) else { return Ok(vec![]) };
+    let Some(layer) = comp.layer(lid) else { return Ok(vec![]) };
+    let (Some(path), Some(pr)) = (layer.props.match_path_of(uid), layer.props.find(uid)) else { return Ok(vec![]) };
+    if !s.state.selected_layers.contains(&lid) {
+        return Ok(vec![]);
+    }
+    let kind = (pr.value.kind_name(), pr.value.components().len());
+    Ok(s.state
+        .selected_layers
+        .iter()
+        .filter(|o| **o != lid)
+        .filter_map(|o| comp.layer(*o))
+        .filter(|l| !l.switches.locked)
+        .filter_map(|l| l.props.prop(&path).filter(|q| (q.value.kind_name(), q.value.components().len()) == kind).map(|q| (l.id, q.uid)))
+        .collect())
+}
+
+/// `p` aimed at property `uid` of layer `lid` instead.
+fn twin_params(p: &Value, lid: LayerId, uid: Uid) -> Value {
+    let mut q = p.clone();
+    if let Some(o) = q.as_object_mut() {
+        o.remove("path");
+        o.remove("selected");
+        o.remove("offset");
+    }
+    q["layer"] = json!(lid.0);
+    q["prop"] = json!(uid);
+    q
+}
+
+/// The value of property `uid` of `lid` at the CTI as numbers (Position with Separate
+/// Dimensions: its X / Y / Z Position).
+fn current_components(s: &Session, cid: ItemId, lid: LayerId, uid: Uid) -> Option<Vec<f64>> {
+    let l = s.project.comp(cid)?.layer(lid)?;
+    let lt = l.layer_time(s.time_of(cid));
+    if let Some(tr) = l.transform()
+        && tr.get("position").is_some_and(|q| q.uid == uid)
+        && tr.get("positionX").is_some()
+    {
+        return Some(["positionX", "positionY", "positionZ"].iter().filter_map(|m| tr.get(m)).map(|q| q.value_at(lt).as_f64()).collect());
+    }
+    Some(l.props.find(uid)?.value_at(lt).components())
+}
+
+/// A JSON number or array of numbers.
+fn json_numbers(v: &Value) -> Option<Vec<f64>> {
+    match v {
+        Value::Number(n) => Some(vec![n.as_f64()?]),
+        Value::Array(a) => a.iter().map(Value::as_f64).collect(),
+        _ => None,
+    }
+}
+
+/// `prop.set`; with `selected`, also on the other selected layers: the same value, or with
+/// `offset` (a scrub) the same change from each layer's own value. One undo step (with `merge`,
+/// one that a continuing scrub keeps folding into).
+fn set_selected(s: &mut Session, p: &Value) -> Result<Value> {
+    let twins = selected_twins(s, p, "prop.set")?;
+    if twins.is_empty() {
+        return set(s, p);
+    }
+    let (cid, lid, uid) = prop_ref(s, p, "prop.set")?;
+    let new = p.get("value").and_then(json_numbers);
+    let delta = match (b_p(p, "offset"), &new, current_components(s, cid, lid, uid)) {
+        (Some(true), Some(n), Some(c)) => Some(n.iter().zip(c.iter().chain(std::iter::repeat(&0.0))).map(|(a, b)| a - b).collect::<Vec<f64>>()),
+        _ => None,
+    };
+    let run = |s: &mut Session| -> Result<Value> {
+        let out = set(s, p)?;
+        for (olid, ouid) in twins {
+            let mut q = twin_params(p, olid, ouid);
+            if let Some(d) = &delta
+                && let Some(c) = current_components(s, cid, olid, ouid)
+            {
+                let v: Vec<f64> = c.iter().enumerate().map(|(i, x)| x + d.get(i).copied().unwrap_or(0.0)).collect();
+                q["value"] = if p.get("value").is_some_and(Value::is_number) { json!(v.first().copied().unwrap_or(0.0)) } else { json!(v) };
+            }
+            // (a layer that can't take it, e.g. a clamped or static property, keeps its value)
+            let _ = set(s, &q);
+        }
+        Ok(out)
+    };
+    if merge_p(p).is_some() { run(s) } else { super::app_more::grouped(s, "Change Property", run) }
+}
+
+/// `prop.toggleAnimation`; with `selected`, the other selected layers' same property follows.
+fn toggle_anim_selected(s: &mut Session, p: &Value) -> Result<Value> {
+    let twins = selected_twins(s, p, "prop.toggleAnimation")?;
+    if twins.is_empty() {
+        return toggle_anim(s, p);
+    }
+    let run = |s: &mut Session| -> Result<Value> {
+        let on = toggle_anim(s, p)?;
+        for (olid, ouid) in twins {
+            let mut q = twin_params(p, olid, ouid);
+            q["value"] = on.clone();
+            let _ = toggle_anim(s, &q);
+        }
+        Ok(on)
+    };
+    // (a stopwatch drag over several rows keeps folding into the step its `merge` names)
+    if merge_p(p).is_some() { run(s) } else { super::app_more::grouped(s, "Toggle Animation", run) }
+}
+
+/// `prop.toggleKey`; with `selected`, the other selected layers' same property gets (or loses)
+/// its keyframe at the current time too.
+fn toggle_key_selected(s: &mut Session, p: &Value) -> Result<Value> {
+    let twins = selected_twins(s, p, "prop.toggleKey")?;
+    if twins.is_empty() {
+        return toggle_key(s, p);
+    }
+    let cid = prop_ref(s, p, "prop.toggleKey")?.0;
+    super::app_more::grouped(s, "Add/Remove Keyframe", |s| {
+        let added = toggle_key(s, p)?;
+        let want = added.as_bool().unwrap_or(true);
+        for (olid, ouid) in twins {
+            let _ = with_prop(s, "Add/Remove Keyframe", None, cid, olid, ouid, |pr, lt| {
+                if key_at(&pr.keys, lt).is_some() != want {
+                    toggle_key_at(pr, lt);
+                }
+                Ok(())
+            });
+        }
+        Ok(added)
+    })
+}
+
 fn set(s: &mut Session, p: &Value) -> Result<Value> {
     let (cid, lid, uid) = prop_ref(s, p, "prop.set")?;
     // Position with Separate Dimensions: write the X/Y/Z Position properties instead.
@@ -1001,13 +1140,13 @@ pub fn specs() -> Vec<CommandSpec> {
             "Set Property Value",
             [],
             None,
-            "{layer?, path|prop, value, time? (layer s by default), timeBase?: layer|comp, merge?}",
+            "{layer?, path|prop, value, time? (layer s by default), timeBase?: layer|comp, merge?, selected? (also the other selected layers), offset? (them by the same change)}",
             has_layers,
-            set
+            set_selected
         ),
-        cmd!("prop.toggleAnimation", "Toggle Stopwatch", [], None, "{layer?, path|prop, value?, merge?}", has_layers, toggle_anim),
+        cmd!("prop.toggleAnimation", "Toggle Stopwatch", [], None, "{layer?, path|prop, value?, merge?, selected?}", has_layers, toggle_anim_selected),
         cmd!("prop.addKey", "Add Keyframe", [], None, "{layer?, path|prop, time? (layer s by default), timeBase?: layer|comp, value?}", has_layers, add_key),
-        cmd!("prop.toggleKey", "Add or Remove Keyframe at Current Time", [], None, "{layer?, path|prop}", has_layers, toggle_key),
+        cmd!("prop.toggleKey", "Add or Remove Keyframe at Current Time", [], None, "{layer?, path|prop, selected?}", has_layers, toggle_key_selected),
         cmd!(
             "keys.toggleTransform",
             "Add or Remove Transform Keyframe",

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);
+}

crates/ui-egui/src/i18n/ui.rs

@@ -405,6 +405,7 @@ const JAPANESE: &[(&str, &str)] = &[
     ("Select a command to change its shortcut.", "ショートカットを変更するコマンドを選択してください。"),
     ("Select a layer to see its effects", "レイヤーを選択するとエフェクトが表示されます"),
     ("Select a layer to see its properties", "レイヤーを選択するとプロパティが表示されます"),
+    ("({}) Selected Objects", "({}) 個の選択されたオブジェクト"),
     (
         "Select a layer with a mask or shape path (or the path property), or Puppet pins.",
         "マスクまたはシェイプパスのあるレイヤー（またはパスプロパティ）か、パペットピンを選択してください。",
@@ -840,6 +841,7 @@ const SIMPLIFIED_CHINESE: &[(&str, &str)] = &[
     ("Select a command to change its shortcut.", "选择一条命令以更改其快捷键。"),
     ("Select a layer to see its effects", "选择图层以查看其效果"),
     ("Select a layer to see its properties", "选择图层以查看其属性"),
+    ("({}) Selected Objects", "({}) 个选定对象"),
     (
         "Select a layer with a mask or shape path (or the path property), or Puppet pins.",
         "请选择带蒙版或形状路径（或路径属性）的图层，或带有人偶定位点的图层。",
@@ -1364,6 +1366,7 @@ const TRADITIONAL_CHINESE: &[(&str, &str)] = &[
     ("Select a command to change its shortcut.", "選取一條命令以更改其快速鍵。"),
     ("Select a layer to see its effects", "選取圖層以查看其效果"),
     ("Select a layer to see its properties", "選取圖層以查看其屬性"),
+    ("({}) Selected Objects", "({}) 個選取的物件"),
     (
         "Select a layer with a mask or shape path (or the path property), or Puppet pins.",
         "請選取帶遮色片或形狀路徑（或路徑屬性）的圖層，或帶有人偶釘選點的圖層。",
@@ -1882,6 +1885,7 @@ const UKRAINIAN: &[(&str, &str)] = &[
     ("Select a command to change its shortcut.", "Виберіть команду, щоб змінити її клавіатурне скорочення."),
     ("Select a layer to see its effects", "Виділіть шар, щоб переглянути його ефекти"),
     ("Select a layer to see its properties", "Виділіть шар, щоб переглянути його властивості"),
+    ("({}) Selected Objects", "Вибрано об’єктів: ({})"),
     (
         "Select a layer with a mask or shape path (or the path property), or Puppet pins.",
         "Виділіть шар із маскою або контуром фігури (або властивість контуру) чи опорні точки деформації.",

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

@@ -180,7 +180,7 @@ fn key_navigator(
             match what {
                 "prev" => actions.push(("time.go".into(), json!({"to": "prevKey", "prop": uid}))),
                 "next" => actions.push(("time.go".into(), json!({"to": "nextKey", "prop": uid}))),
-                _ => actions.push(("prop.toggleKey".into(), json!({"layer": layer.id.0, "prop": uid}))),
+                _ => actions.push(("prop.toggleKey".into(), json!({"layer": layer.id.0, "prop": uid, "selected": true}))),
             }
         }
     }
@@ -241,7 +241,7 @@ fn prop_row(
         );
         app.auto.add(&format!("effectControls.prop.{uid}.stopwatch"), swr, &prop.name);
         if resp.clicked() {
-            actions.push(("prop.toggleAnimation".into(), json!({"layer": layer.id.0, "prop": uid})));
+            actions.push(("prop.toggleAnimation".into(), json!({"layer": layer.id.0, "prop": uid, "selected": true})));
         }
     }
     if prop.is_animated() {
@@ -273,8 +273,11 @@ fn prop_row(
     ui.interact(name_clip, egui::Id::new(("ec-prop-name", uid)), Sense::click());
     app.auto.add(&format!("effectControls.prop.{uid}.name"), name_clip, &prop.name);
     let merge = format!("ec-{uid}");
-    let set =
+    // With several layers selected the edit applies to the same effect property of each (#491).
-        |actions: &mut Actions, v: serde_json::Value| actions.push(("prop.set".into(), json!({"layer": layer.id.0, "prop": uid, "value": v, "merge": merge})));
+    let scrub = ui.ctx().dragged_id().is_some();
+    let set = |actions: &mut Actions, v: serde_json::Value| {
+        actions.push(("prop.set".into(), json!({"layer": layer.id.0, "prop": uid, "value": v, "merge": merge, "selected": true, "offset": scrub})))
+    };
     let twirl_open = |app: &mut EffectcraftApp, ui: &mut egui::Ui| {
         let open = app.ui.fx_slider_open.contains(&uid);
         let tw = Rect::from_center_size(pos2(swr.min.x - 9.0, cy), vec2(10.0, 10.0));

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

@@ -72,6 +72,17 @@ pub fn show(app: &mut EffectcraftApp, ui: &mut egui::Ui, rect: Rect) {
     let mut y = top;
     let mut actions: Actions = vec![];
 
+    // Several layers selected: "(3) Selected Objects", as in After Effects. The rows show the
+    // first layer; the stopwatch, keyframe button and value edits apply to all of them (#355).
+    let selected = app.session.state.selected_layers.iter().filter(|id| comp.layer(**id).is_some()).count();
+    if selected > 1 {
+        let n = selected.to_string();
+        let label = crate::i18n::tr_args("({}) Selected Objects", &[&n]);
+        p.text(pos2(x0, y + ROW_H / 2.0), Align2::LEFT_CENTER, &label, Tokens::semibold(12.0), t.text);
+        app.auto.add("properties.selectedObjects", Rect::from_min_size(pos2(x0, y), vec2(w, ROW_H)), &label);
+        y += ROW_H + 4.0;
+    }
+
     // ---------------------------------------------------------------- Layer Transform
     if let Some(tr) = layer.transform().cloned() {
         section_header(app, ui, &p, x0, w, y, "Layer Transform", "properties.transform");
@@ -218,7 +229,7 @@ fn transform_row(app: &mut EffectcraftApp, ui: &mut egui::Ui, p: &egui::Painter,
                 match what {
                     "prev" => actions.push(("time.go".into(), json!({"to": "prevKey", "prop": uid}))),
                     "next" => actions.push(("time.go".into(), json!({"to": "nextKey", "prop": uid}))),
-                    _ => actions.push(("prop.toggleKey".into(), json!({"layer": lid, "prop": uid}))),
+                    _ => actions.push(("prop.toggleKey".into(), json!({"layer": lid, "prop": uid, "selected": true}))),
                 }
             }
         }
@@ -228,13 +239,18 @@ fn transform_row(app: &mut EffectcraftApp, ui: &mut egui::Ui, p: &egui::Painter,
         icons::paint(p, swr, Icon::Stopwatch, if resp.hovered() { t.text } else { t.text_dim });
         app.auto.add(&format!("{base}.stopwatch"), swr, &pr.name);
         if resp.clicked() {
-            actions.push(("prop.toggleAnimation".into(), json!({"layer": lid, "prop": uid})));
+            actions.push(("prop.toggleAnimation".into(), json!({"layer": lid, "prop": uid, "selected": true})));
         }
     }
     p.text(pos2(r.min.x + 42.0, cy), Align2::LEFT_CENTER, &pr.name, Tokens::ui(12.0), t.text);
     let vx = r.min.x + (r.width() * 0.5).max(130.0);
     let merge = format!("props-{uid}");
-    let set = |actions: &mut Actions, v: serde_json::Value| actions.push(("prop.set".into(), json!({"layer": lid, "prop": uid, "value": v, "merge": merge})));
+    // With several layers selected the edit applies to all: a scrub by the same change, a typed
+    // value as it is.
+    let scrub = ui.ctx().dragged_id().is_some();
+    let set = |actions: &mut Actions, v: serde_json::Value| {
+        actions.push(("prop.set".into(), json!({"layer": lid, "prop": uid, "value": v, "merge": merge, "selected": true, "offset": scrub})))
+    };
     let value = ectx.value(layer, pr);
     let is_scale = pr.name == "Scale";
     let pct = is_scale || pr.name == "Opacity";

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

@@ -2403,7 +2403,7 @@ pub fn show(app: &mut EffectcraftApp, ui: &mut egui::Ui, rect: Rect) {
                         ui_actions.push(UiAct::SetTime(layer.comp_time(k.time)));
                     }
                     if ui.interact(mid, egui::Id::new(("kmid", uid)), Sense::click()).clicked() {
-                        actions.push(("prop.toggleKey".into(), json!({"layer": layer.id.0, "prop": uid})));
+                        actions.push(("prop.toggleKey".into(), json!({"layer": layer.id.0, "prop": uid, "selected": true})));
                     }
                     app.auto.add(&format!("timeline.prop.{uid}.addKey"), mid, "Add or remove keyframe");
                 }
@@ -2426,10 +2426,11 @@ pub fn show(app: &mut EffectcraftApp, ui: &mut egui::Ui, rect: Rect) {
                             actions.push(("prop.setExpression".into(), json!({"layer": layer.id.0, "prop": uid})));
                         }
                     } else if let Some((value, merge)) = switch_gesture(ui, &resp, r, "stopwatch", prop.is_animated()) {
-                        actions.push(("prop.toggleAnimation".into(), json!({"layer": layer.id.0, "prop": uid, "value": value, "merge": merge})));
+                        actions
+                            .push(("prop.toggleAnimation".into(), json!({"layer": layer.id.0, "prop": uid, "value": value, "merge": merge, "selected": true})));
                     } else if resp.clicked() && !resp.clicked_by(egui::PointerButton::Primary) {
                         // Keyboard or accessibility activation.
-                        actions.push(("prop.toggleAnimation".into(), json!({"layer": layer.id.0, "prop": uid})));
+                        actions.push(("prop.toggleAnimation".into(), json!({"layer": layer.id.0, "prop": uid, "selected": true})));
                     }
                 }
                 let name_x = indent + 12.0;
@@ -3510,8 +3511,11 @@ fn value_editor(
     let uid = prop.uid;
     let mut dims = vec![];
     let merge = format!("scrub-{uid}");
+    // With several layers selected the edit applies to all of them (#491): a scrub by the same
+    // change, a typed value as it is.
+    let scrub = ui.ctx().dragged_id().is_some();
     let set = |actions: &mut Vec<(String, serde_json::Value)>, v: serde_json::Value| {
-        actions.push(("prop.set".into(), json!({"layer": layer.id.0, "prop": uid, "value": v, "merge": merge})))
+        actions.push(("prop.set".into(), json!({"layer": layer.id.0, "prop": uid, "value": v, "merge": merge, "selected": true, "offset": scrub})))
     };
     let is_3d = layer.is_3d();
     let mut x = at.x;

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

@@ -84,12 +84,17 @@ enum Gesture {
         /// Layer pixels → comp as the drag began, and its inverse.
         l2c: Mat3,
         inv: Mat3,
+        /// The other selected layers and their scales as the drag began: they scale by the same
+        /// ratio about their own anchor points, as in After Effects (#491).
+        others: Vec<(LayerId, [f64; 3])>,
     },
     Rotate {
         layer: LayerId,
         center: Pos2,
         start_angle: f64,
         start_rot: f64,
+        /// The other selected layers and their rotations: they turn by the same angle.
+        others: Vec<(LayerId, f64)>,
     },
     Anchor {
         layer: LayerId,
@@ -258,6 +263,16 @@ pub(crate) fn fx2c(ctx: &EvalCtx, layer: &Layer) -> Mat3 {
     from_effect_space(ctx, layer, l2c(ctx, layer).0)
 }
 
+/// The selected, unlocked layers of `comp` other than `layer` (when `layer` is one of them): a
+/// handle or Rotation tool drag on one changes them all.
+fn selected_others<'a>(app: &EffectcraftApp, comp: &'a Comp, layer: LayerId) -> Vec<&'a Layer> {
+    let sel = &app.session.state.selected_layers;
+    if !sel.contains(&layer) {
+        return vec![];
+    }
+    sel.iter().filter(|id| **id != layer).filter_map(|id| comp.layer(*id)).filter(|l| !l.switches.locked && l.transform().is_some()).collect()
+}
+
 /// The least homogeneous depth (`w`: the distance in front of a perspective camera in pixels, 1
 /// for 2D layers and orthographic views) a viewer outline is drawn at: points at or behind the
 /// camera would flip through infinity and draw long lines across the viewer (#572).
@@ -1232,6 +1247,7 @@ pub fn show(app: &mut EffectcraftApp, ui: &mut egui::Ui, rect: Rect) {
             }
             Tool::MaskFeather => ov::begin_feather(app, &ectx, &paths, &map, press).map(Gesture::Overlay),
             Tool::Rotate => pick(app, &ectx, cpt, mods.shift).map(|l| {
+                let others = selected_others(app, &comp, l).into_iter().filter_map(|o| Some((o.id, ectx.f(o, o.transform()?, "rotation", 0.0)))).collect();
                 let layer = comp.layer(l).cloned();
                 let center = layer
                     .as_ref()
@@ -1239,7 +1255,13 @@ pub fn show(app: &mut EffectcraftApp, ui: &mut egui::Ui, rect: Rect) {
                     .map(|p| map.to_screen([p[0], p[1]]))
                     .unwrap_or(pos);
                 let rot = layer.as_ref().and_then(|l| l.transform().map(|tr| ectx.f(l, tr, "rotation", 0.0))).unwrap_or(0.0);
-                Gesture::Rotate { layer: l, center, start_angle: ((pos.y - center.y) as f64).atan2((pos.x - center.x) as f64).to_degrees(), start_rot: rot }
+                Gesture::Rotate {
+                    layer: l,
+                    center,
+                    start_angle: ((pos.y - center.y) as f64).atan2((pos.x - center.x) as f64).to_degrees(),
+                    start_rot: rot,
+                    others,
+                }
             }),
             Tool::PanBehind => pick(app, &ectx, cpt, false).and_then(|l| {
                 let layer = comp.layer(l)?;
@@ -1349,6 +1371,10 @@ pub fn show(app: &mut EffectcraftApp, ui: &mut egui::Ui, rect: Rect) {
                             axes: handle_axes(hi),
                             l2c,
                             inv,
+                            others: selected_others(app, &comp, lid)
+                                .into_iter()
+                                .filter_map(|o| Some((o.id, ectx.v3(o, o.transform()?, "scale", [100.0; 3]))))
+                                .collect(),
                         })
                     })
                 } else if let Some(l) = pick(app, &ectx, cpt, mods.shift) {
@@ -1417,23 +1443,29 @@ pub fn show(app: &mut EffectcraftApp, ui: &mut egui::Ui, rect: Rect) {
                     let _ = app.session.execute("prop.set", json!({"layer": lid.0, "path": "transform/position", "value": v, "merge": merge}));
                 }
             }
-            Gesture::Scale { layer, start_scale, start_local, handle, anchor, axes, l2c, inv } => {
+            Gesture::Scale { layer, start_scale, start_local, handle, anchor, axes, l2c, inv, others } => {
                 let lp = inv.apply(gv2(cpt[0], cpt[1]));
                 let uniform = mods.shift && axes == [true, true];
                 let f = drag_ratio(start_local, [lp.x - anchor[0], lp.y - anchor[1]], axes, uniform);
                 // The handle snaps (to the comp's and other layers' edges, corners and centres),
                 // so a layer scales exactly to them, e.g. to the comp's size.
                 let f = snap_handle(f, handle, anchor, &l2c, axes, uniform, |p| vt::snap(app, &ctx, &ectx, &map, &[layer], &[p], mods));
-                let v = [start_scale[0] * f[0], start_scale[1] * f[1], start_scale[2]];
+                for (lid, s0) in std::iter::once((layer, start_scale)).chain(others) {
-                let _ = app.session.execute("prop.set", json!({"layer": layer.0, "path": "transform/scale", "value": v, "merge": merge}));
+                    let v = [s0[0] * f[0], s0[1] * f[1], s0[2]];
+                    let _ = app.session.execute("prop.set", json!({"layer": lid.0, "path": "transform/scale", "value": v, "merge": merge}));
+                }
             }
-            Gesture::Rotate { layer, center, start_angle, start_rot } => {
+            Gesture::Rotate { layer, center, start_angle, start_rot, others } => {
                 let ang = ((pos.y - center.y) as f64).atan2((pos.x - center.x) as f64).to_degrees();
                 let mut r = start_rot + (ang - start_angle);
                 if mods.shift {
                     r = (r / 45.0).round() * 45.0;
                 }
                 let _ = app.session.execute("prop.set", json!({"layer": layer.0, "path": "transform/rotation", "value": r, "merge": merge}));
+                for (lid, r0) in others {
+                    let _ =
+                        app.session.execute("prop.set", json!({"layer": lid.0, "path": "transform/rotation", "value": r0 + (r - start_rot), "merge": merge}));
+                }
             }
             Gesture::Anchor { layer, start_anchor, start_pos, start, inv, l2p, own, anchor_only } => {
                 // Pan Behind snaps the anchor point (to its own layer's box, other layers' features,

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:?}");
+    }
+}

crates/ui-egui/tests/ui_viewer.rs

@@ -1707,6 +1707,32 @@ fn handle_drags_scale_about_the_anchor_and_follow_the_pointer() {
     assert!(close(scale(&h), [162.5, 162.5]), "{:?}", scale(&h));
 }
 
+/// #491: with several layers selected, a handle drag scales all of them by the same ratio, each
+/// about its own anchor point, in one undo step.
+#[test]
+fn handle_drags_scale_every_selected_layer() {
+    let mut h = harness();
+    let layers = h.state().session.active_comp().unwrap().layers.iter().map(|l| l.id).collect::<Vec<_>>();
+    let (box_id, plate) = (layers[0], layers[1]);
+    let scale = |h: &Harness<'_, EffectcraftApp>, id: LayerId| {
+        let l = h.state().session.active_comp().unwrap().layer(id).unwrap().clone();
+        l.props.prop("transform/scale").unwrap().value.as_vec3()
+    };
+    h.state_mut().session.execute("prop.set", json!({"layer": plate.0, "path": "transform/scale", "value": [50, 50, 100]})).unwrap();
+    h.state_mut().session.execute("layer.select", json!({"layers": [box_id.0, plate.0]})).unwrap();
+    h.state_mut().session.execute("view.snapping", json!({"value": false})).unwrap();
+    h.run_steps(2);
+    let undo0 = h.state().session.history.undo.len();
+    // The box's bottom-right corner (360, 220) to (400, 260): twice the box's size.
+    let corner = rect(&h, &format!("viewer.handle.{}.2", box_id.0)).center();
+    let to = screen(&h, [400.0, 260.0]);
+    drag_path(&mut h, &[corner, to], Default::default());
+    let (b, p) = (scale(&h, box_id), scale(&h, plate));
+    assert!((b[0] - 200.0).abs() < 1.0 && (b[1] - 200.0).abs() < 1.0, "{b:?}");
+    assert!((p[0] - 100.0).abs() < 1.0 && (p[1] - 100.0).abs() < 1.0, "the other layer doubles too: {p:?}");
+    assert_eq!(h.state().session.history.undo.len(), undo0 + 1, "one undo step");
+}
+
 /// #252: with snapping on, a dragged handle snaps to the comp's corners and edges (and other
 /// layers'), so a layer scales exactly to the comp; Shift keeps the proportions and still lands
 /// on the comp's size.