test(core): the segmented slide's busy proof starts in time by vjeux · Pull Request #10315 · meta/astryx

vjeux · 2026-11-06T17:20:14Z

Stacked on #10286: only the last commit is this change. It stays a draft until #10286 lands.

Maintainer impact

SegmentedControl's "the slide keeps advancing every frame while the main thread is blocked" (Chromium) could start its block after the slide had settled. The test tapped, waited for the slide, painted the lens and then blocked: three round trips. Under the CI lane's CPU quota with load, that threw BusyWindowTooLateError. The case also counted frames against the frame rate (expectedFrames - 2), which a quota starves.

Intended invariant

Unchanged: the slide advances on the compositor through a main-thread block. No product code changes, and no timeout, retry or skip.

Change

The lens is painted in the marker colour before the tap. An important rule keeps that colour, since the tap's render writes the lens's own.
The page arms blockMainThread(…, {during: control.group}) before the tap. It blocks once the slide has run 16 ms, on the compositor.
The case judges against the harness's busy-window control (startBusyControl, trackControl), until the lens is within 1 px of rest.

Evidence

Every commit named here is evidence, not for landing.

Failure before / success after, in one container at the CI lane's profile (--cpus=4, CI=1, one worker) with eight busy loops: before, the case failed 1 of 10 on main (028e6ca19d) and 1 of 20 on test: watch before the action at 28 first-contact waits #10286's head, each with BusyWindowTooLateError. This change passed 10 of 10.
Mutants fail on the verdict, 3 of 3 without load and 3 of 3 loaded:
- febffa35af: every animation runs on the main thread. The slide moved on 0 of the control's 27 frames.
- 2fd7e98967: every animation holds its first keyframe. The slide moved on 0 of 27.
Product/runtime behavior verified unchanged: a test-only change.
CI impact: none; no workflow changes.

Scope

No intended public API, product behavior, visual direction, or policy change.
Product changes discovered during the work were removed or split.
Public text and artifacts contain no internal Meta context.

Testing

At this head, without load: 5 of 5.
Strict ESLint on the spec, no new problem; core typecheck; Prettier; review:pre, clean.
Open pull requests touching this spec: test: watch before the action at 28 first-contact waits #10286 (the base) and feat(SegmentedControl): under a finger, a press on the selected segment lifts its surface, as UIKit's does #9724. A group change after test: watch before the action at 28 first-contact waits #10286 touches other cases; whichever lands second rebases.

vercel · 2026-11-06T17:20:19Z

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Project	Deployment	Actions	Updated
astryx	Ready	Preview	Nov 7, 2026 10:50am UTC

astracat-bot · 2026-11-06T17:22:15Z

AI review status for this pull request.

Review status	Updated
🟢 Reviewed (for maintainers only)	Nov 7, 2026, 11:05 AM UTC

github-actions · 2026-11-06T17:22:21Z

PR Analysis Report

🔎 Preview · `81551e8`

Storybook · Sandbox · built from this PR's head

Modified components

Component	Size	LoC	Visual · A11y · RTL
SegmentedControl	91.4 kB (0) · 6.8 kB CSS (0)	—	✓ · ✓ · ✓
SegmentedControlItem	16.5 kB (0) · 4.3 kB CSS (0)	—	✓ · ✓ · ✓

Package size: no published package changed; no new dependencies.

API changes: none in this pull request

Committed API report behind this build: cli 19 rows, core 25 rows; run pnpm api:report.

Touched components checked; add ci:run-visual, ci:run-a11y or ci:run-rtl to run the full suites.

_{Generated by PR Enrichment workflow | View full report}

cixzhang · 2026-11-06T17:25:07Z

/approve-spec 7818c09

astracat-bot

Approved pending CI: the SegmentedControl busy-main-thread case now arms its block in the page before the tap and judges the slide against the busy-window control, and the other changes only take watchAnimations before the action. This diff also carries #10286's commit, so it lands after #10286 or together with it.

Non-blocking: paintLensMarker (SegmentedControl.interaction.browser.spec.ts:389-408) no longer has a caller and adds a @typescript-eslint/no-unused-vars warning, so delete it or move the new !important rule into it and call it from the busy case.

CI has not run on this head yet because the pull request is a draft; it already carries ci:run, so marking it ready for review starts CI.

[Automated review]

astracat-bot · 2026-11-06T17:41:13Z

+  // The lens has no element to carry the harness's marker chip, so the test
+  // paints the lens itself in the marker colour before the tap. The tap
+  // writes the lens's own colour, and an important rule keeps the marker's.
+  await control.group.evaluate((group, rgb) => {


Non-blocking: this replaces paintLensMarker (line 394), which now has no caller and adds a @typescript-eslint/no-unused-vars warning. Delete the helper, or move this !important rule into it and call it here.

The block started after three round trips from the test, so on a loaded machine the slide had settled when it began. The page now paints the lens first and arms the block before the tap. The proof judges the slide against the busy-window control, not against the frame rate.

cixzhang · 2026-11-07T04:17:30Z

/approve-spec 2e7a318

cixzhang · 2026-11-07T07:19:25Z

/approve-spec 5e02c9e

astracat-bot

Approved pending CI: the SegmentedControl busy-main-thread case now arms its block in the page before the tap and judges the slide against the busy-window control, so the block no longer starts after the slide has settled on a loaded runner.

Regression (non-blocking): after main's rework of this file's other lens cases, the busy case was the last caller of waitForLens (SegmentedControl.interaction.browser.spec.ts:228-282), so the helper is now unused and adds a @typescript-eslint/no-unused-vars warning; delete it with the waitForAnimation and AnimationWatch imports that only it uses.
Non-blocking: paintLensMarker (SegmentedControl.interaction.browser.spec.ts:364-383) still has no caller and adds the same warning, so delete it too.

CI has not run on this head because the pull request is a draft. It carries ci:run, so marking it ready for review starts CI.

[Automated review]

astracat-bot · 2026-11-07T07:29:38Z

+  });
  await control.tapSegment('table');
-  // Running and composited before the block.
-  const [slide] = await waitForLens(control, {minProgressMs: 16, watch});


Regression (non-blocking): main has since moved this file's other lens cases to holds from the slide's start, so this was the last call to waitForLens (now line 232 of the head). The helper is unused after this change and adds a second @typescript-eslint/no-unused-vars warning. Delete it, together with the waitForAnimation and type AnimationWatch imports (lines 60 and 66) that only it uses.

cixzhang · 2026-11-07T10:44:54Z

/approve-spec 81551e8

astracat-bot

Approved: the SegmentedControl busy-main-thread case now arms its block in the page before the tap and judges the slide against the busy-window control, so a loaded runner can no longer start the block after the slide has settled. On this head the case passes in CI's Chromium interaction lane with the lens moving on every frame the control moves, and CI is green.

Optional, non-blocking:

waitForLens and paintLensMarker have no callers after this change (noted inline); deleting them, with the waitForAnimation and AnimationWatch imports only waitForLens uses, clears two lint warnings.
Now that blockMainThread takes pseudoElement, passing pseudoElement: '::before' would let only the lens start the block, not another animation under the radiogroup such as a segment's press release.

[Automated review]

meta-cla Bot added the CLA Signed This label is managed by the Meta Open Source bot. label Nov 6, 2026

cixzhang added the ci:run A maintainer lets this outside contributor's pull request run CI (AST-030 FR17) label Nov 6, 2026

vercel Bot deployed to Preview November 6, 2026 17:27

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vjeux force-pushed the test/segmented-slide-busy-start branch from 7818c09 to 2e7a318 Compare November 7, 2026 04:11

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Merge branch 'main' into test/segmented-slide-busy-start

5e02c9e

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vjeux marked this pull request as ready for review November 7, 2026 07:59

vjeux requested review from cixzhang, imdreamrunner and josephfarina as code owners November 7, 2026 07:59

Merge branch 'main' into test/segmented-slide-busy-start

81551e8

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cixzhang merged commit f1ad44f into meta:main Nov 7, 2026
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vjeux deleted the test/segmented-slide-busy-start branch November 7, 2026 12:41

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vjeux commented Nov 6, 2026

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