[modacker]

What broke

packages/webui/test/browser/provider-form.spec.mjs — "with motion allowed › the provider picker animates in and out" — failed on CI with:

Expected: "webui-dropdown-out"
Received: ""

It is a pre-existing flake on the baseline, not a regression from a change: it hit PR #230's run 39865540489 (job 120126683759) and PR #224's ubuntu verify (run 39868998671, job 120138436238), while other runs of the same baseline passed.

Why it failed

The exit animation is a 120ms window. The popup's class flips to is-leaving — the class the stylesheet hangs webui-dropdown-out on — and 120ms later the skeleton unmounts the popup.

The assertion read that window by polling a live DOM node across the process boundary:

await expect
  .poll(() => listbox.evaluate((node) => getComputedStyle(node).animationName).catch(() => "gone"))
  .toBe("webui-dropdown-out");

expect.poll samples at 0ms, 100ms, 350ms, 850ms… — a grid coarser than the window it is looking for. So a sample could land:

• before the popup started leaving → it reads the entrance animation (webui-dropdown-in);
• after the popup was unmounted → the locator resolves the node just before the unmount and the evaluation then reads it detached, and a detached node's computed style is empty for every property → "".

Both outcomes fail the assertion while the popup was never in the wrong state. The empty string is exactly what a detached node reports (Chromium, measured):

attached: 'out'   detached: ''   detachedDuration: ''   detachedDisplay: ''

The var() in the exit duration is not involved: --webui-dropdown-exit-duration resolves to 120ms, and the computed animation on the leaving popup is webui-dropdown-out at 0.12s.

What this changes

The reading moves into the page, to where the state changes. A MutationObserver on the popup's class records the popup's own computed animation-name at the moment that class changes, and the test reads that record. The record outlives the popup, so the sample can neither miss the window nor read a node that is already gone.

The expectation is unchanged: the popup carries the exit animation rather than the entrance one while it fades out. A failure now shows the recorded state instead of an empty string.

How it was checked

• The flake, reproduced against this branch's baseline (webui @ 63b9739): playwright test packages/webui/test/browser/provider-form.spec.mjs -g "animates in and out" --repeat-each=30 → 7 failed, 23 passed. Five of the failures read Received: "" and one read Received: "webui-dropdown-in" — the two ends of the same race.
• With the fix: the same command with --repeat-each=50 → 50 passed.
• playwright test packages/webui/test/browser/provider-form.spec.mjs → 8 passed.
• playwright test packages/webui/test/browser → 160 passed, 7 skipped, 1 failed. The failure is native-runtime.spec.mjs ("native PTYs survive another page, reload and a full five-slot roster"), which dies with posix_spawnp failed. — it spawns a real PTY and fails the same way with the pre-fix spec, so it is this machine's environment, not this change.
• The assertion still has teeth: with the exit animation broken in the built stylesheet (animation: none on .webui-dropdown-popup.is-leaving), the case fails with the recorded state — animationName: "none" instead of "webui-dropdown-out".

Not addressed here

The exit animation is cut short by roughly a frame: the stylesheet runs it for --webui-dropdown-exit-duration (120ms) while the skeleton unmounts the popup 120ms after the class flips, and the animation itself starts on the next frame. That shortens the fade cosmetically; it is not a flake, and it is left for a separate change.