fix(studio): keep timeline selection authoritative in the preview sync

The store-to-preview sync no longer applies a partial selection: if a resolvable member's
DOM node is not ready yet it bails and retries on the next effect run, so the write-back can
never shrink the store's selection by dropping an unresolved member.

Marquee row hit-testing reuses shouldShowTimelineLayerGroupHeader instead of re-deriving
the group-header placement rule, keeping one owner for that predicate.

packages/studio/src/hooks/useTimelineSelectionPreviewSync.ts

@@ -88,13 +88,20 @@ export function useTimelineSelectionPreviewSync({
     let cancelled = false;
     const syncSelection = async () => {
       const selections: DomEditSelection[] = [];
+      let resolvableCount = 0;
       for (const id of selectedIds) {
         const element = timelineElements.find((item) => (item.key ?? item.id) === id);
         if (!element) continue;
+        resolvableCount += 1;
         const selection = await buildDomSelectionForTimelineElement(element);
         if (selection) selections.push(selection);
       }
       if (cancelled) return;
+      // The store is the source of truth: applying a partial set would write that
+      // shrunk set back and silently drop the members whose DOM node was not ready.
+      // Bail instead; a later effect run (on timelineElements/DOM change) applies the
+      // full set once every resolvable member has a live node.
+      if (selections.length < resolvableCount) return;
       if (selections.length === 0) {
         applyDomSelection(null, { revealPanel: false });
       } else if (selections.length === 1) {

packages/studio/src/player/components/timelineEditing.ts

@@ -2,6 +2,7 @@ import { formatTime } from "../lib/time";
 import { roundToCenti } from "../../utils/rounding";
 import type { StackingTimelineLayer, TimelineLayerId } from "./timelineTrackOrder";
 import { resolveTimelineLayerStackingMove } from "./timelineLayerDrag";
+import { shouldShowTimelineLayerGroupHeader } from "./TimelineLayerGroupHeader";
 import type { TimelineStackingElement, TimelineStackingReorderIntent } from "./timelineStacking";
 
 import {
@@ -348,7 +349,7 @@ export function selectTimelineElementsInMarquee({
   for (const layerId of layerOrder) {
     const layer = layerById.get(layerId);
     if (!layer) continue;
-    if (layer.contextKey !== "" && layer.contextKey !== previousContextKey) {
+    if (shouldShowTimelineLayerGroupHeader(layer.contextKey, previousContextKey)) {
       rowTop += groupHeaderHeight;
     }
     const rowBottom = rowTop + trackHeight;