test(perf): end the settle window at its scheduled deadline

A cap or quiet timer delayed by a busy main thread used the moment it ran
as the settle point, so shifts after the 3 s cap could enter the settled
counter. The settle point is now the deadline the timer was scheduled for.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/electron-backend-e2e/src/performance/journey-renderer-probe.spec.ts

@@ -526,6 +526,37 @@ test('the cap ends the settle window while the workspace content keeps mutating'
     assert.doesNotThrow(() => assertJourneyRendererProbeState(state));
 });
 
+test('a late cap timer ends the window at its scheduled deadline', async () => {
+    const fixture = createFixture({
+        settle: { ...FAST_SETTLE, capMs: 100, quietMs: 60_000 },
+    });
+    const [observer] = fixture.observers as [FakeObserver];
+    const now = () => fixture.window.performance.now();
+    renderFirstCard(fixture);
+    await waitFor(() => fixture.rawState().final);
+    const cutoffMs =
+        (fixture.rawState().firstCardPaintEpochMs ?? 0) -
+        fixture.window.performance.timeOrigin;
+    observer.emit([layoutShift(now(), 0.25)]);
+    // Hold the main thread past the cap so its timer runs late, and shift
+    // the layout after the deadline while it is held.
+    while (now() < cutoffMs + 250) {
+        // busy
+    }
+    observer.emit([layoutShift(now(), 0.5)]);
+    observer.queue.push(layoutShift(now(), 1));
+    await waitFor(() => fixture.rawState().settle.status !== 'pending');
+    const state = fixture.state();
+    assert.equal(state.settle.status, 'cap');
+    const settledAfterCutoff =
+        (state.settle.epochMs ?? 0) - (state.firstCardPaintEpochMs ?? 0);
+    assert.ok(
+        settledAfterCutoff >= 100 && settledAfterCutoff < 150,
+        `settle point at the deadline, not when the timer ran (${settledAfterCutoff} ms)`
+    );
+    assert.equal(state.counters.layoutShiftScoreSettled, 0.25);
+});
+
 test('watches the document element when the settle root is missing', async () => {
     const fixture = createFixture({
         settle: { ...FAST_SETTLE, rootSelector: 'app-missing-root' },

apps/electron-backend-e2e/src/performance/journey-renderer-probe.ts

@@ -421,12 +421,17 @@ export function journeyRendererProbeScript(
         };
     };
     // Starts at the first-card cutoff. Every mutation record under the root
-    // restarts the quiet timer; the cap timer never moves.
+    // restarts the quiet timer; the cap timer never moves. A timer can run
+    // late on a busy main thread, so the settle point is the deadline it was
+    // scheduled for, never the moment it ran: a late timer must not let
+    // shifts after the deadline into the counter.
     const startSettle = (settle: JourneyRendererProbeSettleOptions) => {
         const root = document.querySelector(settle.rootSelector);
         state.settle.observedTarget =
             root === null ? 'documentElement' : 'root';
         let quietTimer: ReturnType<typeof setTimeout> | undefined;
+        let quietDeadlineEpochMs = Number.POSITIVE_INFINITY;
+        const capDeadlineEpochMs = epoch() + settle.capMs;
         const capTimer = setTimeout(() => end('cap'), settle.capMs);
         const settleObserver = new MutationObserver((records) => {
             if (state.settle.status !== 'pending') return;
@@ -440,10 +445,13 @@ export function journeyRendererProbeScript(
             clearTimeout(capTimer);
             state.settle.domMutations += settleObserver.takeRecords().length;
             settleObserver.disconnect();
-            endSettle(status, epoch());
+            const deadline =
+                status === 'cap' ? capDeadlineEpochMs : quietDeadlineEpochMs;
+            endSettle(status, Math.min(epoch(), deadline));
         };
         const armQuiet = (): void => {
             clearTimeout(quietTimer);
+            quietDeadlineEpochMs = epoch() + settle.quietMs;
             quietTimer = setTimeout(() => end('quiet'), settle.quietMs);
         };
         settleObserver.observe(root ?? document.documentElement ?? document, {

docs/architecture/performance-journeys.md

@@ -129,9 +129,11 @@ The settle window opens at the first-card cutoff. A second
 content pane (the document element if it is missing, reported in
 `evidence.settle.observedTarget`). The window closes when nothing in that
 subtree has mutated for 500 ms, or 3 s after the cutoff, whichever comes
-first. The settle point is sampled when the timer fires, and every entry
-that starts before it counts, including entries still queued in the
-observer. Why this point:
+first. The settle point is the deadline the firing timer was scheduled for
+(the last mutation plus 500 ms, or the cutoff plus 3 s), or the moment it
+ran if that is earlier, so a timer delayed by a busy main thread does not
+let later shifts in. Every entry that starts at or before the settle point
+counts, including entries still queued in the observer. Why this point:
 
 - A DOM change in the content pane is what causes the shifts this counter
   is after (data resolving, skeletons swapped for content), so quiet in that