fix: cancel DismissibleLayer afterSleep timer on destroy (#2080)

[dieison-depra]

Context

Hi maintainers 👋

Thank you for Bits UI — we rely on it heavily for Dialog / Sheet / Popover surfaces in a Svelte 5 app, and we are grateful for the care that goes into the headless layer.

While working through browser-test noise on our side, we kept hitting Svelte’s derived_inert warning when floating layers unmounted quickly. That led us to #2080, which already describes the race clearly. We would like to offer a small, focused patch as a proposal — happy to iterate if you prefer a different shape.

Root cause (what we validated)

In DismissibleLayerState (use-dismissable-layer.svelte.ts), setup schedules:

afterSleep(1, () => {
  if (!this.opts.ref.current) return;
  globalThis.bitsDismissableLayers.set(this, this.#behaviorType);
  unsubEvents = this.#addEventListeners();
});

What we observed when a layer is destroyed within that ~1 ms window (common under vitest-browser cleanup, rapid navigation, or CPU contention):

1. onDestroyEffect runs while unsubEvents is still a no-op (the real subscribe has not happened yet).
2. The pending afterSleep timer is not cancelled.
3. ~1 ms later the callback still runs on a destroyed instance and reads this.opts.ref.current → Svelte emits derived_inert.
4. If that read is still truthy, document-level pointerdown listeners can be attached to a dead layer and never cleaned up — aligning with the comment on DismissibleLayer: derived_inert warning — afterSleep(1) timer reads ref.current after the layer is destroyed #2080 about reopened dialogs self-closing ~10 ms later (stale onInteractOutside + debounce).

We confirmed stacks in a real SvelteKit + vitest-browser suite (bits-ui@2.18.1) converging on:

• get current (svelte-toolbelt Box / ref derived)
• #markResponsibleLayer / the deferred attach path
• document-level pointerdown

This is the same family of teardown race already documented in #2080; we are not claiming a new root cause, only re-validating it end-to-end before suggesting a fix.

Proposed change

Minimal lifecycle hygiene, aligned with the suggested fix in #2080 and with patterns already used elsewhere in this repo (e.g. clearing timers in Select / Popover / Menu):

1. Keep a handle to the afterSleep timer (pendingTimer).
2. Cancel it in both cleanup() and onDestroyEffect.
3. Set a destroyed flag on teardown and short-circuit on destroyed before any this.opts.ref.current read (so JS never touches the destroyed derived).

No public API changes. No behavior change for the happy path (layer lives longer than 1 ms).

Tests

Added two browser regressions under tests/src/tests/dialog/dialog.browser.test.ts:

• unmount while open → assert console.warn does not contain derived_inert
• close → immediate pointer reopen → dialog stays open ≥ 50 ms and no derived_inert

Local verification (after pnpm build:packages):

cd tests
pnpm exec vitest run --project=browser --browser=chromium   src/tests/dialog/dialog.browser.test.ts

Result: 43 passed | 1 todo (full dialog browser file, Chromium).

Note: this environment cannot run WebKit (macOS 12 + Playwright limitation), so CI coverage on WebKit will need your pipeline — happy to adjust if anything fails there.

We also exercised the same patch against our app’s LookupModal / ResponsiveSheet vitest-browser suite (open → unmount, open → close → open) with zero derived_inert warnings after the change.

Invitation / next steps

This is offered as a collaborative starting point, not a demand that the fix must land exactly this way. If you would rather:

• extract a shared “cancel deferred work on destroy” helper,
• guard additional call sites in the same family (Avatar / floating / link-preview were mentioned in related issues), or
• reshape the tests,

we are very happy to follow your guidance and update the PR.

Closes #2080

Thank you for considering this, and for the excellent library 🙏

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[leemthai]

Hit this too — confirmed independently in a Svelte 5 game project using Dialog. One interesting extra detail: the warnings genuinely compound over a long session — each subsequent click/focus event anywhere in the app seems to re-trigger every stale listener accumulated so far, so a single click later on can produce 15-20 warnings at once, not just one. Would be great to see #2087 merged - thanks for the fix!