diff --git a/apps/sandbox/src/app/(sandbox)/pages/motion-lab/LabDemos.tsx b/apps/sandbox/src/app/(sandbox)/pages/motion-lab/LabDemos.tsx
index 8738bab87e88b..1162b4dcc981c 100644
--- a/apps/sandbox/src/app/(sandbox)/pages/motion-lab/LabDemos.tsx
+++ b/apps/sandbox/src/app/(sandbox)/pages/motion-lab/LabDemos.tsx
@@ -40,6 +40,12 @@ import {
 } from './proposed/TabList';
 import {Skeleton} from '@astryxdesign/core/Skeleton';
 import {Token} from '@astryxdesign/core/Token';
+import {
+  TopNav,
+  TopNavHeading,
+  TopNavMegaMenu,
+  TopNavMegaMenuItem,
+} from '@astryxdesign/core/TopNav';
 import {HStack, VStack} from '@astryxdesign/core/Layout';
 import {Text} from '@astryxdesign/core/Text';
 import {useMotionLab} from './MotionLabStore';
@@ -897,6 +903,111 @@ export function OriginRig({kind}: {kind: 'zero' | 'centre' | 'anchored'}) {
   );
 }
 
+// --- TopNav mega-menu exit ----------------------------------------------------
+
+const MEGA_MENU_LOOP_MS = 3200;
+const MEGA_MENU_OPEN_MS = 1200;
+
+/**
+ * The real TopNav mega-menu, opened and closed on a loop through its own
+ * trigger. Before is core as it ships: the panel rises 4px and fades on open
+ * and close, and does the same under Reduce Motion. After is the same
+ * component with the missing branch as a sandbox prototype
+ * (MotionLab.module.css `.megaMenuProposed`): under Reduce Motion the panel
+ * fades within 200 ms on a touch screen and changes at once with a mouse
+ * (design:reduced-motion DR2, DR3); the lab's Degrade shows the fade, its
+ * Delete the cut.
+ * The two panes open half a loop apart: opening one auto popover closes any
+ * other, so in step they would close each other.
+ */
+export function TopNavExitRig({mode}: {mode: Mode}) {
+  const {reducedMotion} = useMotionLab();
+  const rootRef = useRef<HTMLDivElement>(null);
+  const timersRef = useRef<number[]>([]);
+  const setOpen = useCallback((isOpen: boolean) => {
+    const trigger = rootRef.current?.querySelector<HTMLButtonElement>(
+      'button[aria-expanded]',
+    );
+    if (trigger == null || (trigger.ariaExpanded === 'true') === isOpen) {
+      return;
+    }
+    // A pointer-style click (detail 1): `click()` sends detail 0, which the
+    // menu reads as Enter or Space and answers by moving focus into the
+    // panel. Focus stays wherever the person left it.
+    const focused = document.activeElement;
+    trigger.dispatchEvent(
+      new MouseEvent('click', {
+        bubbles: true,
+        cancelable: true,
+        detail: 1,
+        view: window,
+      }),
+    );
+    const moved = document.activeElement;
+    if (moved === focused || !(moved instanceof HTMLElement)) {
+      return;
+    }
+    if (focused instanceof HTMLElement && focused !== document.body) {
+      focused.focus({preventScroll: true});
+    } else {
+      // Nothing had focus: leave nothing focused.
+      moved.blur();
+    }
+  }, []);
+  useLoop(() => {
+    timersRef.current.forEach(window.clearTimeout);
+    const offset = mode === 'before' ? 0 : MEGA_MENU_LOOP_MS / 2;
+    timersRef.current = [
+      window.setTimeout(() => setOpen(true), offset),
+      window.setTimeout(() => setOpen(false), offset + MEGA_MENU_OPEN_MS),
+    ];
+  }, MEGA_MENU_LOOP_MS);
+  useEffect(() => () => timersRef.current.forEach(window.clearTimeout), []);
+
+  return (
+    <VStack gap={2} {...stylex.props(sx.full)}>
+      <div
+        ref={rootRef}
+        className={mode === 'after' ? styles.megaMenuProposed : undefined}
+        data-reduced-motion={mode === 'after' ? reducedMotion : undefined}>
+        <TopNav
+          label={mode === 'before' ? 'Today' : 'Proposed'}
+          heading={<TopNavHeading heading="Acme" />}
+          startContent={
+            <TopNavMegaMenu
+              label="Products"
+              items={
+                <>
+                  <TopNavMegaMenuItem
+                    title="Analytics"
+                    description="Track behavior across your apps"
+                    href="#analytics"
+                  />
+                  <TopNavMegaMenuItem
+                    title="Security"
+                    description="Protection for your data"
+                    href="#security"
+                  />
+                  <TopNavMegaMenuItem
+                    title="Automation"
+                    description="Workflows that run themselves"
+                    href="#automation"
+                  />
+                </>
+              }
+            />
+          }
+        />
+      </div>
+      <Hint>
+        {mode === 'before'
+          ? 'An exit that travels, and the proof the pattern works: the panel rises 4px and fades on open and close. Under Reduce Motion it does exactly the same.'
+          : 'The same exit with the missing branch: under Reduce Motion the panel fades within 200 ms on a touch screen and changes at once with a mouse; the lab\u2019s Degrade shows the fade, Delete the cut. Turn either on to compare.'}
+      </Hint>
+    </VStack>
+  );
+}
+
 // --- loops -------------------------------------------------------------------
 
 export function LoopRig({policy}: {policy: 'today' | 'delete' | 'degrade'}) {
diff --git a/apps/sandbox/src/app/(sandbox)/pages/motion-lab/MotionLab.module.css b/apps/sandbox/src/app/(sandbox)/pages/motion-lab/MotionLab.module.css
index 382edee8ac3f5..6a68c91986276 100644
--- a/apps/sandbox/src/app/(sandbox)/pages/motion-lab/MotionLab.module.css
+++ b/apps/sandbox/src/app/(sandbox)/pages/motion-lab/MotionLab.module.css
@@ -753,6 +753,38 @@
   }
 }
 
+/* ------------------------------------------------- TopNav mega-menu ---- */
+
+/* A sandbox prototype of the reduced-motion branch TopNav's mega-menu panel
+   lacks. Under Reduce Motion it never rises: on a touch screen it only
+   fades, no longer than the 200 ms cross-fade, and with a mouse it changes
+   at once (design:reduced-motion DR1-DR3). The panel stays a DOM descendant
+   in the top layer, so the scope reaches it. The lab's Degrade shows the
+   fade; Delete cuts. */
+@media (prefers-reduced-motion: reduce) and (pointer: coarse) {
+  .megaMenuProposed [popover] {
+    transform: none !important;
+    transition-duration: min(var(--duration-medium-min), 200ms) !important;
+  }
+}
+
+@media (prefers-reduced-motion: reduce) and (not (pointer: coarse)) {
+  .megaMenuProposed [popover] {
+    transform: none !important;
+    transition-duration: 0s !important;
+  }
+}
+
+.megaMenuProposed[data-reduced-motion='degrade'] [popover] {
+  transform: none !important;
+  transition-duration: min(var(--duration-medium-min), 200ms) !important;
+}
+
+.megaMenuProposed[data-reduced-motion='delete'] [popover] {
+  transform: none !important;
+  transition-duration: 0s !important;
+}
+
 /* ---------------------------------------------------------------- tint ---- */
 
 /* The Table hover tint, at whatever duration and curve is being argued over.
diff --git a/apps/sandbox/src/app/(sandbox)/pages/motion-lab/previewPlan.ts b/apps/sandbox/src/app/(sandbox)/pages/motion-lab/previewPlan.ts
index a46433a4a131d..d87f804111edf 100644
--- a/apps/sandbox/src/app/(sandbox)/pages/motion-lab/previewPlan.ts
+++ b/apps/sandbox/src/app/(sandbox)/pages/motion-lab/previewPlan.ts
@@ -571,7 +571,7 @@ export const PREVIEW_PLAN: ReadonlyArray<PreviewRow> = [
     motionCost: 'idle',
     dirs: ['TopNav'],
     today: 'Mega-menu with a real exit, sub-lists, chevrons.',
-    gap: 'The only component animating a popover exit, and it never checks reduced motion.',
+    gap: 'Its panel exit travels, and it never checks reduced motion.',
     shows:
       'Its exit as the existing proof the pattern works, plus the missing reduced-motion branch.',
     hardCase:
@@ -582,8 +582,8 @@ export const PREVIEW_PLAN: ReadonlyArray<PreviewRow> = [
       'reduced motion',
       'traverse speed',
     ],
-    status: 'specified',
-    rig: null,
+    status: 'built',
+    rig: 'TopNavExitRig',
     priority: 1,
     measurementNote: null,
   },
diff --git a/apps/sandbox/src/app/(sandbox)/pages/motion-lab/previews/page.tsx b/apps/sandbox/src/app/(sandbox)/pages/motion-lab/previews/page.tsx
index 593cf55a52701..6c096bc531f2f 100644
--- a/apps/sandbox/src/app/(sandbox)/pages/motion-lab/previews/page.tsx
+++ b/apps/sandbox/src/app/(sandbox)/pages/motion-lab/previews/page.tsx
@@ -30,6 +30,7 @@ import {
   SkeletonSwapRig,
   StatCountRig,
   TabIndicatorRig,
+  TopNavExitRig,
 } from '../LabDemos';
 
 export default function PreviewsPage() {
@@ -196,6 +197,34 @@ export default function PreviewsPage() {
         </DemoBody>
       </DemoCard>
 
+      <DemoCard
+        title="TopNav mega-menu exit"
+        question="An anchored panel whose exit travels, and it never checks Reduce Motion."
+        badges={<Badge variant="warning" label="PARTIAL" />}>
+        <ComparePanes
+          panes={[
+            {
+              tone: 'before',
+              label: 'Today — the same rise under Reduce Motion',
+              content: <TopNavExitRig mode="before" />,
+            },
+            {
+              tone: 'after',
+              label: 'Proposed — Reduce Motion never rises',
+              content: <TopNavExitRig mode="after" />,
+            },
+          ]}
+        />
+        <DemoBody>
+          <Text type="supporting" color="secondary">
+            Both panes are core&rsquo;s real <code>TopNav</code>. The right one
+            carries the missing reduced-motion branch as a sandbox prototype in
+            CSS. Animator retargeting would come with the proposed public{' '}
+            <code>useAnimator</code> (AST-107, spec PR #8600).
+          </Text>
+        </DemoBody>
+      </DemoCard>
+
       <DemoCard
         title="Chip add and remove"
         question="Tokenizer, Token and FileInput share this. Removal is the hard half: the chip has to leave while its neighbours close the gap."