diff --git a/apps/docs/editor/custom-ui/content-controls.mdx b/apps/docs/editor/custom-ui/content-controls.mdx
index 1ca5dea4a3..101c3e57d3 100644
--- a/apps/docs/editor/custom-ui/content-controls.mdx
+++ b/apps/docs/editor/custom-ui/content-controls.mdx
@@ -40,6 +40,7 @@ The event tells you *what* is active; `getRect` tells you *where* to draw. `acti
 | Read one control | `ui.contentControls.get({ id })` |
 | Position your UI | `ui.contentControls.getRect({ id })` |
 | Scroll a control into view | `ui.contentControls.scrollIntoView({ id })` |
+| Re-anchor your UI when the page moves | `ui.viewport.observe(() => ...)` |
 | Hover and right-click hit-testing | `ui.viewport.entityAt()` / `contextAt()` |
 | Change content, tags, or locks | `editor.doc.contentControls.*` |
 
@@ -47,9 +48,10 @@ The event tells you *what* is active; `getRect` tells you *where* to draw. `acti
 
 `scrollIntoView` resolves the control's position from the document, so it works even when the control is on a page that hasn't rendered yet (the page mounts, then scrolls). It scrolls only - it does not move the cursor into the control.
 
+`ui.viewport.observe` is the single signal for "your `getRect()` coordinates may be stale, re-query": it fires (coalesced, once per frame) on scroll, resize, zoom, and layout reflow, so an overlay anchored with `getRect` stays glued without hand-wiring those events yourself.
+
 ## Current limits
 
-- No geometry-change subscription. Re-read `getRect()` on scroll, resize, and the `pagination-update` / `zoomChange` events.
 - No focus-by-id helper. Clicking a control in the document still drives selection.
 
 ## See also
diff --git a/demos/__tests__/contract-templates-chip-anchor.spec.ts b/demos/__tests__/contract-templates-chip-anchor.spec.ts
new file mode 100644
index 0000000000..cf16ec6558
--- /dev/null
+++ b/demos/__tests__/contract-templates-chip-anchor.spec.ts
@@ -0,0 +1,73 @@
+import { test, expect } from '@playwright/test';
+
+/**
+ * SD-3311 regression: the field chip must stay anchored to its active control
+ * after a geometry change that fires NO scroll event (zoom). The chip is a
+ * fixed-position overlay positioned from `ui.contentControls.getRect()`. Today
+ * field-chip only re-anchors on active-change / scroll / resize, so a zoom
+ * leaves it stranded (verified: ~230px drift). This is RED until
+ * `ui.viewport.observe()` lands and field-chip re-queries on it.
+ *
+ * Runs only for the contract-templates demo (the shared suite runs once per DEMO).
+ */
+
+test.use({ viewport: { width: 1280, height: 800 } });
+
+test('field chip stays anchored to its control after a zoom (no-scroll geometry change)', async ({ page }) => {
+  test.skip(process.env.DEMO !== 'contract-templates', 'contract-templates demo only');
+
+  await page.route('**/ingest.superdoc.dev/**', (r) =>
+    r.fulfill({ status: 204, contentType: 'application/json', body: '{}' }),
+  );
+  await page.goto('/');
+  await page.waitForFunction(
+    () => {
+      const ui = (window as any).__demo?.state?.ui;
+      return !!ui && ui.contentControls.getSnapshot().items.length > 0;
+    },
+    null,
+    { timeout: 30_000 },
+  );
+
+  // Activate the first inline smart field so the chip appears and anchors.
+  await page.waitForSelector('.superdoc-structured-content-inline[data-sdt-id]');
+  await page.locator('.superdoc-structured-content-inline[data-sdt-id]').first().click();
+  await page.locator('.sd-field-chip').waitFor({ state: 'visible', timeout: 10_000 });
+
+  // Horizontal gap between the chip's left edge and its active control's left
+  // edge. positionChip sets chip.left = control.left, so this is ~0 when anchored.
+  const probe = () =>
+    page.evaluate(() => {
+      const ui = (window as any).__demo.state.ui;
+      const activeId = ui.contentControls.getSnapshot().activeId as string | null;
+      const chip = document.querySelector<HTMLElement>('.sd-field-chip');
+      const ctrl = activeId ? document.querySelector<HTMLElement>(`[data-sdt-id="${activeId}"]`) : null;
+      if (!chip || !ctrl) return null;
+      const c = chip.getBoundingClientRect();
+      const k = ctrl.getBoundingClientRect();
+      return { dxLeft: Math.abs(c.left - k.left), ctrlLeft: Math.round(k.left) };
+    });
+
+  const before = await probe();
+  expect(before, 'chip + active control both resolve').not.toBeNull();
+  expect(before!.dxLeft, 'chip starts anchored to the control').toBeLessThanOrEqual(2);
+
+  // Zoom: a geometry change with no scroll event.
+  await page.evaluate(() => (window as any).__demo.superdoc.setZoom(150));
+
+  // Poll for the settled state: the control has moved (zoom applied) AND the
+  // chip has re-anchored to it. Polling absorbs the rAF/repaint delay between
+  // the geometry change and the viewport.observe -> positionChip re-query.
+  // Without the SD-3311 fix this stays "drift:~230" and times out.
+  await expect
+    .poll(
+      async () => {
+        const p = await probe();
+        if (!p) return 'no-probe';
+        if (p.ctrlLeft === before!.ctrlLeft) return 'control-not-moved';
+        return p.dxLeft <= 2 ? 'anchored' : `drift:${Math.round(p.dxLeft)}`;
+      },
+      { timeout: 6_000 },
+    )
+    .toBe('anchored');
+});
diff --git a/demos/contract-templates/src/field-chip.ts b/demos/contract-templates/src/field-chip.ts
index 53b328e5e6..a6311ce52e 100644
--- a/demos/contract-templates/src/field-chip.ts
+++ b/demos/contract-templates/src/field-chip.ts
@@ -115,7 +115,11 @@ export function attachFieldChip(superdoc: SuperDoc, ui: SuperDocUI, lookup: Smar
     positionChip();
   };
 
-  const onScrollOrResize = () => positionChip();
+  // Re-anchor whenever the viewport geometry changes. ui.viewport.observe is
+  // the single signal for this - it fires on scroll, resize, zoom, and
+  // layout/pagination reflow, so we catch the zoom / reflow cases that
+  // hand-wired window scroll + resize listeners miss (SD-3311).
+  const onViewportChange = () => positionChip();
 
   // SD-3232: the active control comes from the public SuperDoc event. The
   // payload includes the SdtRef (id + tag), so we can narrow to smart
@@ -150,13 +154,11 @@ export function attachFieldChip(superdoc: SuperDoc, ui: SuperDocUI, lookup: Smar
   };
 
   superdoc.on('content-control:active-change', onActiveChange);
-  window.addEventListener('scroll', onScrollOrResize, true);
-  window.addEventListener('resize', onScrollOrResize);
+  const unobserveViewport = ui.viewport.observe(onViewportChange);
 
   return () => {
     superdoc.off('content-control:active-change', onActiveChange);
-    window.removeEventListener('scroll', onScrollOrResize, true);
-    window.removeEventListener('resize', onScrollOrResize);
+    unobserveViewport();
     chipEl.remove();
   };
 }
diff --git a/packages/super-editor/src/ui/create-super-doc-ui.ts b/packages/super-editor/src/ui/create-super-doc-ui.ts
index 2af4ece7d6..e6bbd5efeb 100644
--- a/packages/super-editor/src/ui/create-super-doc-ui.ts
+++ b/packages/super-editor/src/ui/create-super-doc-ui.ts
@@ -66,6 +66,7 @@ import type {
   ViewportPositionAtInput,
   ViewportPositionHit,
   ViewportHandle,
+  ViewportGeometryEvent,
   ViewportRect,
   ViewportRectResult,
 } from './types.js';
@@ -958,6 +959,81 @@ export function createSuperDocUI(options: SuperDocUIOptions): SuperDocUI {
     };
   };
 
+  // --- Viewport geometry-invalidation signal (ui.viewport.observe) ---------
+  // One "your cached getRect() coords may be stale, re-query" notification.
+  // Sources: layout/pagination repaints (post-paint), zoom, and DOM scroll /
+  // resize. rAF-coalesced, so a burst collapses to one notification per frame.
+  const geometryListeners = new Set<(event: ViewportGeometryEvent) => void>();
+  const pendingGeometryReasons = new Set<Exclude<ViewportGeometryEvent['reason'], 'mixed'>>();
+  let geometryRaf: number | null = null;
+  let zoomPending = false;
+
+  const cancelGeometryFrame = () => {
+    if (geometryRaf == null) return;
+    if (typeof cancelAnimationFrame === 'function') cancelAnimationFrame(geometryRaf);
+    else clearTimeout(geometryRaf as unknown as ReturnType<typeof setTimeout>);
+    geometryRaf = null;
+  };
+  const flushGeometry = () => {
+    geometryRaf = null;
+    const reasons = [...pendingGeometryReasons];
+    pendingGeometryReasons.clear();
+    if (geometryListeners.size === 0 || reasons.length === 0) return;
+    const reason: ViewportGeometryEvent['reason'] = reasons.length === 1 ? reasons[0] : 'mixed';
+    [...geometryListeners].forEach((listener) => {
+      try {
+        listener({ reason });
+      } catch {
+        // Isolate a faulty consumer; the others still get notified.
+      }
+    });
+  };
+  const scheduleGeometry = (reason: Exclude<ViewportGeometryEvent['reason'], 'mixed'>) => {
+    if (geometryListeners.size === 0) return;
+    pendingGeometryReasons.add(reason);
+    if (geometryRaf != null) return;
+    geometryRaf =
+      typeof requestAnimationFrame === 'function'
+        ? requestAnimationFrame(flushGeometry)
+        : (setTimeout(flushGeometry, 0) as unknown as number);
+  };
+  // zoomChange fires *before* the re-render, so notifying then would hand
+  // consumers stale rects. Tag the next post-paint layout flush as 'zoom'.
+  const onGeometryZoom = () => {
+    zoomPending = true;
+  };
+  const onGeometryLayout = () => {
+    if (zoomPending) {
+      zoomPending = false;
+      scheduleGeometry('zoom');
+    } else {
+      scheduleGeometry('layout');
+    }
+  };
+  const onWindowScrollGeometry = () => scheduleGeometry('scroll');
+  const onWindowResizeGeometry = () => scheduleGeometry('resize');
+  let domGeometryAttached = false;
+  const attachDomGeometryListeners = () => {
+    if (domGeometryAttached || typeof window === 'undefined') return;
+    domGeometryAttached = true;
+    // Capture phase so scrolls inside the editor's own scroll container
+    // (scroll events don't bubble) are still observed.
+    window.addEventListener('scroll', onWindowScrollGeometry, true);
+    window.addEventListener('resize', onWindowResizeGeometry);
+  };
+  const detachDomGeometryListeners = () => {
+    if (!domGeometryAttached || typeof window === 'undefined') return;
+    domGeometryAttached = false;
+    window.removeEventListener('scroll', onWindowScrollGeometry, true);
+    window.removeEventListener('resize', onWindowResizeGeometry);
+  };
+  teardown.push(() => {
+    detachDomGeometryListeners();
+    cancelGeometryFrame();
+    geometryListeners.clear();
+    pendingGeometryReasons.clear();
+  });
+
   // Wire SuperDoc-instance events. The wrapper-side bus (editorCreate /
   // document-mode-change / zoomChange) is the only path for some of
   // these signals today; if the wrapper migrates them to the editor
@@ -966,8 +1042,12 @@ export function createSuperDocUI(options: SuperDocUIOptions): SuperDocUI {
     SUPERDOC_EVENTS.forEach((name) => {
       superdoc.on?.(name, scheduleNotify);
     });
+    // zoom drives geometry (post-paint, tagged via onGeometryLayout) — separate
+    // from the slice recompute that SUPERDOC_EVENTS triggers.
+    superdoc.on?.('zoomChange', onGeometryZoom);
     teardown.push(() => {
       SUPERDOC_EVENTS.forEach((name) => superdoc.off?.(name, scheduleNotify));
+      superdoc.off?.('zoomChange', onGeometryZoom);
     });
   }
 
@@ -1089,8 +1169,18 @@ export function createSuperDocUI(options: SuperDocUIOptions): SuperDocUI {
     PRESENTATION_EVENTS.forEach((name) => {
       next.on?.(name, onPresentationChange);
     });
+    // Geometry-only: layout repaints move painted rects without a body
+    // `transaction`. Drive the viewport geometry signal, NOT the slice
+    // recompute (which would re-attach editor listeners on every repaint).
+    // Listen to `layoutUpdated` only: `paginationUpdate` is emitted
+    // back-to-back with the same payload for the same paint
+    // (PresentationEditor.ts:6491-6492), so subscribing to both would
+    // double-count one repaint — a zoom would coalesce to 'mixed' instead of
+    // 'zoom'. `layoutUpdated` alone covers every repaint.
+    next.on?.('layoutUpdated', onGeometryLayout);
     currentPresentationTeardown = () => {
       PRESENTATION_EVENTS.forEach((name) => next.off?.(name, onPresentationChange));
+      next.off?.('layoutUpdated', onGeometryLayout);
     };
   };
 
@@ -1927,6 +2017,21 @@ export function createSuperDocUI(options: SuperDocUIOptions): SuperDocUI {
       };
     },
 
+    observe(listener: (event: ViewportGeometryEvent) => void): () => void {
+      geometryListeners.add(listener);
+      // Attach the DOM scroll/resize listeners only while someone is observing.
+      if (geometryListeners.size === 1) attachDomGeometryListeners();
+      return () => {
+        if (!geometryListeners.delete(listener)) return;
+        if (geometryListeners.size === 0) {
+          detachDomGeometryListeners();
+          cancelGeometryFrame();
+          pendingGeometryReasons.clear();
+          zoomPending = false;
+        }
+      };
+    },
+
     async scrollIntoView(input: ScrollIntoViewInput): Promise<ScrollIntoViewOutput> {
       return runScrollIntoView(input);
     },
diff --git a/packages/super-editor/src/ui/types.ts b/packages/super-editor/src/ui/types.ts
index a5c394a892..b1bac10f5e 100644
--- a/packages/super-editor/src/ui/types.ts
+++ b/packages/super-editor/src/ui/types.ts
@@ -714,11 +714,10 @@ export interface SuperDocUI {
   selection: SelectionHandle;
 
   /**
-   * Viewport domain — imperative geometry queries for sticky-card /
-   * floating-toolbar placement against painted entities and ranges.
-   * No subscription substrate — viewport rects are read on-demand by
-   * the consumer (e.g. on hover, on scroll, on layout-change events
-   * the consumer already listens to). Browser-only by definition.
+   * Viewport domain — geometry queries for sticky-card / floating-toolbar
+   * placement against painted entities and ranges, plus
+   * {@link ViewportHandle.observe} to learn when those rects may have moved.
+   * Browser-only by definition.
    */
   viewport: ViewportHandle;
 
@@ -1831,17 +1830,38 @@ export type ViewportRectResult =
     };
 
 /**
- * Imperative viewport-geometry surface. No subscription primitive —
- * rects are read on demand. Consumers who need to reflow on layout
- * change typically already listen to a `transaction` / `paint` /
- * `scroll` event upstream and call `getRect` from there.
+ * Reason a {@link ViewportHandle.observe} notification fired. `'mixed'`
+ * when more than one change coalesced into the same animation frame.
  */
+export type ViewportGeometryReason = 'layout' | 'zoom' | 'scroll' | 'resize' | 'mixed';
+
+/**
+ * Payload for {@link ViewportHandle.observe}. Intentionally minimal: the
+ * signal means "your cached `getRect()` coordinates may be stale, re-query" -
+ * it carries no geometry.
+ */
+export interface ViewportGeometryEvent {
+  reason: ViewportGeometryReason;
+}
+
 export interface ViewportHandle {
   /**
    * Look up the painted rectangle(s) of an entity or text range in
    * viewport coordinates. Synchronous — no DOM mutation required.
    */
   getRect(input: ViewportGetRectInput): ViewportRectResult;
+  /**
+   * Subscribe to viewport geometry invalidation. The listener fires (once
+   * per animation frame, coalesced) after anything that can move painted
+   * rectangles: layout / pagination repaints, zoom, and DOM scroll / resize.
+   * It carries no coordinates — re-query {@link getRect} for the entities you
+   * care about. Returns an unsubscribe.
+   *
+   * This is the single signal overlays should listen to instead of
+   * hand-wiring scroll + resize + layout + zoom (and still missing cases like
+   * reflow and zoom, which fire no scroll event).
+   */
+  observe(listener: (event: ViewportGeometryEvent) => void): () => void;
   /**
    * Scroll the viewport so the target is visible. Browser-only by
    * definition: drives `presentation.navigateTo()` for entity targets
diff --git a/packages/super-editor/src/ui/viewport.test.ts b/packages/super-editor/src/ui/viewport.test.ts
index e3706880c3..5ec9ee543b 100644
--- a/packages/super-editor/src/ui/viewport.test.ts
+++ b/packages/super-editor/src/ui/viewport.test.ts
@@ -555,3 +555,137 @@ describe('ui.viewport.contextAt - bundle composition', () => {
     ui.destroy();
   });
 });
+
+describe('ui.viewport.observe — geometry invalidation (SD-3311)', () => {
+  // The rAF flush resolves within a frame; a short real-timer wait covers both
+  // the requestAnimationFrame path and the setTimeout fallback.
+  const nextFrame = () => new Promise((resolve) => setTimeout(resolve, 30));
+
+  it('fires once per frame on scroll (reason "scroll") and stops after unsubscribe', async () => {
+    const { superdoc } = makeStubs();
+    const ui = createSuperDocUI({ superdoc });
+    const events: Array<{ reason: string }> = [];
+    const unsubscribe = ui.viewport.observe((e) => events.push(e));
+
+    // Burst in one frame -> a single coalesced notification.
+    window.dispatchEvent(new Event('scroll'));
+    window.dispatchEvent(new Event('scroll'));
+    await nextFrame();
+    expect(events).toEqual([{ reason: 'scroll' }]);
+
+    unsubscribe();
+    window.dispatchEvent(new Event('scroll'));
+    await nextFrame();
+    expect(events).toHaveLength(1); // no notification after unsubscribe
+
+    ui.destroy();
+  });
+
+  it('coalesces different reasons in the same frame to "mixed"', async () => {
+    const { superdoc } = makeStubs();
+    const ui = createSuperDocUI({ superdoc });
+    const events: Array<{ reason: string }> = [];
+    ui.viewport.observe((e) => events.push(e));
+
+    window.dispatchEvent(new Event('scroll'));
+    window.dispatchEvent(new Event('resize'));
+    await nextFrame();
+    expect(events).toEqual([{ reason: 'mixed' }]);
+
+    ui.destroy();
+  });
+
+  it('does not notify after the UI is destroyed', async () => {
+    const { superdoc } = makeStubs();
+    const ui = createSuperDocUI({ superdoc });
+    const events: Array<{ reason: string }> = [];
+    ui.viewport.observe((e) => events.push(e));
+
+    ui.destroy();
+    window.dispatchEvent(new Event('resize'));
+    await nextFrame();
+    expect(events).toEqual([]);
+  });
+});
+
+// Stub with real event emitters so tests can drive the engine signals that
+// feed ui.viewport.observe: superdoc `zoomChange` and presentation
+// `layoutUpdated` / `paginationUpdate`.
+function makeEmitter() {
+  const map = new Map<string, Set<(p?: unknown) => void>>();
+  return {
+    on: (e: string, h: (p?: unknown) => void) => {
+      if (!map.has(e)) map.set(e, new Set());
+      map.get(e)!.add(h);
+    },
+    off: (e: string, h: (p?: unknown) => void) => {
+      map.get(e)?.delete(h);
+    },
+    emit: (e: string, p?: unknown) => [...(map.get(e) ?? [])].forEach((h) => h(p)),
+  };
+}
+
+function makeGeometryStub() {
+  const sd = makeEmitter();
+  const pres = makeEmitter();
+  const emptyList = () => ({ evaluatedRevision: 'r1', total: 0, items: [], page: { limit: 0, offset: 0, returned: 0 } });
+  const editor: { on: ReturnType<typeof vi.fn>; off: ReturnType<typeof vi.fn>; doc: unknown; presentationEditor: unknown } = {
+    on: vi.fn(),
+    off: vi.fn(),
+    doc: {
+      selection: { current: vi.fn(() => ({ empty: true })) },
+      comments: { list: vi.fn(emptyList) },
+      trackChanges: { list: vi.fn(emptyList) },
+      contentControls: { list: vi.fn(() => ({ items: [], total: 0 })) },
+    },
+    presentationEditor: undefined,
+  };
+  editor.presentationEditor = {
+    on: pres.on,
+    off: pres.off,
+    getActiveEditor: () => editor,
+    getEntityRects: vi.fn(() => []),
+    navigateTo: vi.fn(async () => true),
+  };
+  const superdoc: SuperDocLike = {
+    activeEditor: editor as never,
+    config: { documentMode: 'editing' },
+    on: sd.on as never,
+    off: sd.off as never,
+  };
+  return { superdoc, emitSuperdoc: sd.emit, emitPresentation: pres.emit };
+}
+
+describe('ui.viewport.observe — repaint reason (SD-3311 regression)', () => {
+  const nextFrame = () => new Promise((resolve) => setTimeout(resolve, 30));
+
+  it('reports "zoom" for a zoom repaint, not "mixed" (layoutUpdated + paginationUpdate are one paint)', async () => {
+    const { superdoc, emitSuperdoc, emitPresentation } = makeGeometryStub();
+    const ui = createSuperDocUI({ superdoc });
+    const events: Array<{ reason: string }> = [];
+    ui.viewport.observe((e) => events.push(e));
+
+    // zoomChange (pre-paint), then the paired post-paint repaint events.
+    emitSuperdoc('zoomChange');
+    emitPresentation('layoutUpdated');
+    emitPresentation('paginationUpdate'); // same paint / payload — must not double-count
+    await nextFrame();
+
+    expect(events).toEqual([{ reason: 'zoom' }]);
+    ui.destroy();
+  });
+
+  it('reports "layout" for a plain repaint (the paginationUpdate alias does not make it "mixed")', async () => {
+    const { superdoc, emitPresentation } = makeGeometryStub();
+    const ui = createSuperDocUI({ superdoc });
+    const events: Array<{ reason: string }> = [];
+    ui.viewport.observe((e) => events.push(e));
+
+    emitPresentation('layoutUpdated');
+    emitPresentation('paginationUpdate');
+    await nextFrame();
+
+    expect(events).toEqual([{ reason: 'layout' }]);
+    ui.destroy();
+  });
+});