diff --git a/.planning/NOTES.md b/.planning/NOTES.md
index 06379cd..1cf0812 100644
--- a/.planning/NOTES.md
+++ b/.planning/NOTES.md
@@ -3891,3 +3891,40 @@ scan takes a minute and is worth running before demoting anybody's links
 — `explore-album-details`'s tracklist (number / title / artist /
 duration) is the one that plausibly *is* mostly link, and is the one
 #5 is about to add selection to.
+
+## A layout is still moving when a guard says it has arrived (measured 2026-08-20)
+
+`album-dropdown.spec.ts` failed with `Expected 80, Received 10` twice
+over two sessions, and #133 already strengthened its guard from
+"scrollable at all" to "has at least the range the assertion needs".
+That was necessary and could not be sufficient, and the reason is
+structural rather than a matter of thresholds: **a guard and the write
+it guards are separate CDP round trips**, so the page is free to
+re-lay-out between them. Polling harder cannot close a window between
+two moments; only removing the window can.
+
+Measured directly, sampling `scrollHeight - clientHeight` on
+`.grid-scroll-container` every frame across a 1440x900 → 900x600 resize,
+three runs:
+
+| t (ms) | range |
+|---|---|
+| 0 | 0 |
+| 1 | **88** |
+| 8–14 | 330 (settled) |
+
+88 satisfies a guard asking for 80 and is not the settled value, so the
+guard can pass while the grid is one layout pass from done. Under
+full-suite load the transient is worse — the observed failure had 10 —
+which is why it shows up on the second run of a suite and not in ten
+consecutive runs of the file alone (0/10 both before and after the fix).
+
+The shape to write instead: **one page-side call that performs the
+action and returns what it observes**, with `expect.poll` retrying
+*that*. `scrollTo()` sets `scrollTop` and returns `scrollTop`, so the
+assertion is about what the grid did rather than about what it was
+ready to do. `layout-overflow.spec.ts`'s sidebar probe already had the
+fused half and was missing the retry; it has both now.
+
+Worth generalising: a spec that resizes and then measures is asserting
+about a moving target for the next dozen frames. Fuse, then poll.
diff --git a/e2e/specs/album-dropdown.spec.ts b/e2e/specs/album-dropdown.spec.ts
index c629b62..9ade4a1 100644
--- a/e2e/specs/album-dropdown.spec.ts
+++ b/e2e/specs/album-dropdown.spec.ts
@@ -110,27 +110,30 @@ test.describe('the album dropdown', () => {
     await app.setViewportSize({ width: 900, height: 600 });
 
     try {
-      // Wait for the range the assertion below actually needs, not for
-      // "scrollable at all" (#133). The guard used to be
-      // `scrollHeight > clientHeight + 40` while the next line asks to
-      // reach 80, so any range in 41-79 satisfied it and could not
-      // satisfy the assertion — and the grid passes through exactly
-      // that while it settles, because it recomputes its columns after
-      // the resize rather than during it. The settled range here is
-      // 330, so this waits rather than weakening anything.
+      // The container has to be a scroller at all, which is the thing
+      // the defect behind this spec broke and is a property rather
+      // than a moment.
       await expect
         .poll(() => scrollRange(app))
-        .toMatchObject({ room: true, overflowY: 'auto' });
+        .toMatchObject({ overflowY: 'auto' });
 
-      await app.evaluate((target) => {
-        const sc = document
-          .querySelector('cover-grid')
-          ?.shadowRoot?.querySelector('.grid-scroll-container');
-
-        if (sc) sc.scrollTop = target;
-      }, SCROLL_TARGET);
-
-      expect(await scrollTop(app)).toBe(SCROLL_TARGET);
+      // **Scrolling it and reading it back are one round trip** (#151).
+      //
+      // #133 made the guard ask for the range this needs rather than
+      // for "scrollable at all", which was necessary and is not
+      // sufficient: a guard and the write it guards are separate
+      // `evaluate` calls, so the grid can satisfy the range and settle
+      // out of it before the write lands. It still does — observed as
+      // `Expected 80, Received 10` in the second of three consecutive
+      // full-suite runs, with the spec green alone on the same app
+      // straight afterwards.
+      //
+      // Polling harder cannot close a window between two moments; only
+      // removing the window can. So the probe sets `scrollTop` and
+      // returns what it reads back, in one page-side call, and the
+      // poll retries *that* — which also means the assertion is about
+      // what the grid did rather than about what it was ready to do.
+      await expect.poll(() => scrollTo(app, SCROLL_TARGET)).toBe(SCROLL_TARGET);
 
       // And the dropdown it opens is on screen, wherever the manager
       // decides that leaves the scroll. It is *not* "the position is
@@ -261,7 +264,7 @@ async function closeDropdown(app: Page): Promise<void> {
   });
 }
 
-/** Whether the grid can scroll at all, which decides if a probe can move. */
+/** Whether the grid is a scroller at all, which is what the bug broke. */
 async function scrollRange(app: Page) {
   return app.evaluate((target) => {
     const sc = document
@@ -269,22 +272,37 @@ async function scrollRange(app: Page) {
       ?.shadowRoot?.querySelector('.grid-scroll-container');
 
     return {
-      // `room` is the precondition of the assertion that follows it:
-      // enough range to actually reach the target. A threshold below
-      // what the caller depends on is not a guard.
+      // Reported for the failure message rather than waited on: `room`
+      // was the guard #133 strengthened, and #151 is that a guard in
+      // its own round trip cannot speak for the write in the next one.
+      // `scrollTo` below is the assertion now; this says *why* it did
+      // not reach the target when it does not.
       room: !!sc && sc.scrollHeight - sc.clientHeight >= target,
       overflowY: sc ? getComputedStyle(sc).overflowY : '',
     };
   }, SCROLL_TARGET);
 }
 
-async function scrollTop(app: Page): Promise<number> {
-  return app.evaluate(
-    () =>
-      document
-        .querySelector('cover-grid')
-        ?.shadowRoot?.querySelector('.grid-scroll-container')?.scrollTop ?? -1,
-  );
+/**
+ * Scroll the grid and report where it actually landed, in one call.
+ *
+ * The whole point is that the set and the read share a moment: a
+ * `scrollTop` write is clamped to the range *at the instant it lands*,
+ * so reading it back in a second round trip asks a container that may
+ * have re-laid out in between.
+ */
+async function scrollTo(app: Page, target: number): Promise<number> {
+  return app.evaluate((to) => {
+    const sc = document
+      .querySelector('cover-grid')
+      ?.shadowRoot?.querySelector('.grid-scroll-container');
+
+    if (!sc) return -1;
+
+    sc.scrollTop = to;
+
+    return sc.scrollTop;
+  }, target);
 }
 
 /** Whether the open dropdown is inside the scroll container's viewport. */
diff --git a/e2e/specs/layout-overflow.spec.ts b/e2e/specs/layout-overflow.spec.ts
index eaa1953..9815b49 100644
--- a/e2e/specs/layout-overflow.spec.ts
+++ b/e2e/specs/layout-overflow.spec.ts
@@ -132,22 +132,34 @@ test.describe('the app fits in its own window', () => {
     // be dragged here, but a scaled display or a large system font can
     // still land the layout in it, and clipping the nav with no scroll
     // is the failure that made Settings unreachable.
-    const reachable = await app.locator('app-sidebar').evaluate((el) => {
-      const settings = el.shadowRoot?.querySelector<HTMLElement>(
-        '[data-testid="nav-settings"]',
-      );
+    //
+    // The scroll and the measurement share one `evaluate` — #151's
+    // rule, which this already had — and the whole probe is polled,
+    // which it did not: a viewport change settles asynchronously, so a
+    // single attempt reads whatever the sidebar happened to be doing.
+    // The probe is safe to repeat because scrolling to the bottom twice
+    // is scrolling to the bottom.
+    await expect
+      .poll(() =>
+        app.locator('app-sidebar').evaluate((el) => {
+          const settings = el.shadowRoot?.querySelector<HTMLElement>(
+            '[data-testid="nav-settings"]',
+          );
 
-      if (!settings) return null;
+          if (!settings) return null;
 
-      el.scrollTop = el.scrollHeight;
+          el.scrollTop = el.scrollHeight;
 
-      const item = settings.getBoundingClientRect();
-      const pane = el.getBoundingClientRect();
+          const item = settings.getBoundingClientRect();
+          const pane = el.getBoundingClientRect();
 
-      return item.bottom <= Math.ceil(pane.bottom) && item.top >= Math.floor(pane.top);
-    });
-
-    expect(reachable).toBe(true);
+          return (
+            item.bottom <= Math.ceil(pane.bottom) &&
+            item.top >= Math.floor(pane.top)
+          );
+        }),
+      )
+      .toBe(true);
   });
 });