diff --git a/packages/studio/src/hooks/domSelectionTimelineMirror.ts b/packages/studio/src/hooks/domSelectionTimelineMirror.ts
index 27dcdc2b15..2531f441bb 100644
--- a/packages/studio/src/hooks/domSelectionTimelineMirror.ts
+++ b/packages/studio/src/hooks/domSelectionTimelineMirror.ts
@@ -28,6 +28,7 @@ export function announceTimelineSelection(
   deps: TimelineMirrorDeps,
   group: DomEditSelection[],
   primary: DomEditSelection | null,
+  replaceSet = false,
 ): void {
   const {
     timelineElements,
@@ -72,10 +73,10 @@ export function announceTimelineSelection(
     }
     return;
   }
-  // A late async primary that already belongs to the live set must preserve the
-  // group. A fresh single click does not belong to it, so publish the singleton
-  // first; otherwise `preserveSet` clears the set and sync wipes the canvas.
-  if (group.length > 1 || !getTimelineSelectionSet().has(timelineAnchor)) {
+  // A late async primary inside the live set keeps the group. A click outside it, or a
+  // group the caller states in full (`replaceSet`: shift toggle, marquee, timeline sync), is published
+  // first; otherwise `preserveSet` clears or keeps a stale set and sync undoes the canvas.
+  if (replaceSet || group.length > 1 || !getTimelineSelectionSet().has(timelineAnchor)) {
     setTimelineSelectionSet(publishedMembers);
   }
   setSelectedTimelineElementId(timelineAnchor, { preserveSet: true });
diff --git a/packages/studio/src/hooks/useDomSelection.test.ts b/packages/studio/src/hooks/useDomSelection.test.ts
index dda61c7e97..43047c25f6 100644
--- a/packages/studio/src/hooks/useDomSelection.test.ts
+++ b/packages/studio/src/hooks/useDomSelection.test.ts
@@ -29,9 +29,13 @@ function renderHarness(
   cleanup: () => void;
   timeline: TimelineSpies;
 } {
+  // Reads back what was last published, as the timeline store does.
+  let publishedSet: ReadonlySet<string> = new Set();
   const timeline: TimelineSpies = {
     setSelectedTimelineElementId: vi.fn(),
-    setTimelineSelectionSet: vi.fn(),
+    setTimelineSelectionSet: vi.fn((ids: Set<string>) => {
+      publishedSet = ids;
+    }),
   };
   const host = document.createElement("div");
   document.body.append(host);
@@ -47,7 +51,7 @@ function renderHarness(
       captionEditMode: false,
       previewIframeRef: { current: null },
       timelineElements: options.timelineElements ?? [],
-      getTimelineSelectionSet: () => new Set(),
+      getTimelineSelectionSet: () => publishedSet,
       setSelectedTimelineElementId: timeline.setSelectedTimelineElementId,
       setTimelineSelectionSet: timeline.setTimelineSelectionSet,
       setRightCollapsed: vi.fn(),
@@ -107,6 +111,17 @@ function timelineElement(domId: string): TimelineElement {
   } as TimelineElement;
 }
 
+function renderCardAndChip() {
+  const card = makeSelection("Card", Object.assign(document.createElement("div"), { id: "card" }));
+  const chip = makeSelection("Chip", Object.assign(document.createElement("div"), { id: "chip" }));
+  document.body.append(card.element, chip.element);
+  const harness = renderHarness(
+    { activeCompPath: "index.html", projectId: "project-1", refreshKey: 0 },
+    { timelineElements: [timelineElement("card"), timelineElement("chip")] },
+  );
+  return { harness, card, chip };
+}
+
 /**
  * A marquee builds the group correctly and then used to lose it: it announced only
  * the primary to the timeline, the timeline is the source of truth for what is
@@ -116,24 +131,9 @@ function timelineElement(domId: string): TimelineElement {
  */
 describe("useDomSelection marquee", () => {
   it("announces every marquee'd element to the timeline, anchored on the primary", () => {
-    const first = document.createElement("div");
-    first.id = "card";
-    const second = document.createElement("div");
-    second.id = "chip";
-    document.body.append(first, second);
-    const harness = renderHarness(
-      { activeCompPath: "index.html", projectId: "project-1", refreshKey: 0 },
-      { timelineElements: [timelineElement("card"), timelineElement("chip")] },
-    );
+    const { harness, card, chip } = renderCardAndChip();
 
-    act(() =>
-      harness
-        .current()
-        .applyMarqueeSelection(
-          [makeSelection("Card", first), makeSelection("Chip", second)],
-          false,
-        ),
-    );
+    act(() => harness.current().applyMarqueeSelection([card, chip], false));
 
     expect(harness.current().domEditGroupSelections).toHaveLength(2);
     expect(harness.timeline.setTimelineSelectionSet).toHaveBeenCalledWith(
@@ -181,20 +181,10 @@ describe("useDomSelection marquee", () => {
  */
 describe("useDomSelection additive", () => {
   it("announces both members when a second element joins the selection", () => {
-    const first = document.createElement("div");
-    first.id = "card";
-    const second = document.createElement("div");
-    second.id = "chip";
-    document.body.append(first, second);
-    const harness = renderHarness(
-      { activeCompPath: "index.html", projectId: "project-1", refreshKey: 0 },
-      { timelineElements: [timelineElement("card"), timelineElement("chip")] },
-    );
+    const { harness, card, chip } = renderCardAndChip();
 
-    act(() => harness.current().applyDomSelection(makeSelection("Card", first)));
-    act(() =>
-      harness.current().applyDomSelection(makeSelection("Chip", second), { additive: true }),
-    );
+    act(() => harness.current().applyDomSelection(card));
+    act(() => harness.current().applyDomSelection(chip, { additive: true }));
 
     expect(harness.current().domEditGroupSelections).toHaveLength(2);
     expect(harness.timeline.setTimelineSelectionSet).toHaveBeenLastCalledWith(
@@ -205,6 +195,40 @@ describe("useDomSelection additive", () => {
     });
     harness.cleanup();
   });
+
+  // The timeline syncs its set back onto the canvas, so a set left at two re-adds the member.
+  it.each([
+    ["shift+click removes a member", "toggle"],
+    ["a marquee catches one member of a live group", "marquee"],
+  ] as const)("publishes the smaller set when %s", (_, shrink) => {
+    const { harness, card, chip } = renderCardAndChip();
+
+    act(() => harness.current().applyMarqueeSelection([card, chip], false));
+    act(() =>
+      shrink === "toggle"
+        ? harness.current().applyDomSelection(chip, { additive: true })
+        : harness.current().applyMarqueeSelection([card], false),
+    );
+
+    expect(harness.current().domEditGroupSelections).toHaveLength(1);
+    expect(harness.timeline.setTimelineSelectionSet).toHaveBeenLastCalledWith(new Set(["card"]));
+    expect(harness.timeline.setSelectedTimelineElementId).toHaveBeenLastCalledWith("card", {
+      preserveSet: true,
+    });
+    harness.cleanup();
+  });
+
+  it("keeps the live set when a single member is re-announced", () => {
+    const { harness, card, chip } = renderCardAndChip();
+
+    act(() => harness.current().applyMarqueeSelection([card, chip], false));
+    act(() => harness.current().applyDomSelection(chip));
+
+    expect(harness.timeline.setTimelineSelectionSet).toHaveBeenLastCalledWith(
+      new Set(["card", "chip"]),
+    );
+    harness.cleanup();
+  });
 });
 
 describe("useDomSelection", () => {
diff --git a/packages/studio/src/hooks/useDomSelection.ts b/packages/studio/src/hooks/useDomSelection.ts
index 76ca4bca94..4802a6937e 100644
--- a/packages/studio/src/hooks/useDomSelection.ts
+++ b/packages/studio/src/hooks/useDomSelection.ts
@@ -83,7 +83,7 @@ export function useDomSelection({
   // ── Callbacks ──
 
   const announceTimelineSelection = useCallback(
-    (group: DomEditSelection[], primary: DomEditSelection | null) =>
+    (group: DomEditSelection[], primary: DomEditSelection | null, replaceSet?: boolean) =>
       announceSelectionToTimeline(
         {
           timelineElements,
@@ -93,6 +93,7 @@ export function useDomSelection({
         },
         group,
         primary,
+        replaceSet,
       ),
     [
       getTimelineSelectionSet,
@@ -179,7 +180,7 @@ export function useDomSelection({
             setRightPanelTab("design");
           }
         }
-        announceTimelineSelection(nextGroup, nextSelection);
+        announceTimelineSelection(nextGroup, nextSelection, isAdditiveSelection);
         return;
       }
 
@@ -510,7 +511,7 @@ export function useDomSelection({
       domEditGroupSelectionsRef.current = nextGroup;
       setDomEditSelection(nextSelection);
       setDomEditGroupSelections(nextGroup);
-      announceTimelineSelection(nextGroup, nextSelection);
+      announceTimelineSelection(nextGroup, nextSelection, true);
     },
     [applyDomSelection, announceTimelineSelection],
   );