fix(react-tag-picker): cancel the aside-width frame in a cleanup, not the effect body - #36667
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… the effect body
useTagPickerControl cancels the ResizeObserver callback's pending
requestAnimationFrame from the effect body rather than its cleanup. The effect
runs once on mount (measured: 1 of 1 on every load); whether it cancels is decided
by whether React's passive-effect flush lands before or after the observer's first
callback has stored a raf id. When it lands after, the only write of
--fui-TagPickerControl-aside-width is cancelled before it can fire, the control's
padding-inline-end falls back to spacingHorizontalM, and the absolutely positioned
aside overlays the trigger.
Measured over 12 identical fresh page loads of one TagPicker, production build:
3 of 12 wrote the token; resting padding-inline-end was 30px or 12px and the
trigger's input 258.297 or 276.297 -- an 18px width swing between runs of the same
page. With the cancel moved into the effect's cleanup, 12 of 12 write it and both
values are single-valued (30px, 258.297).
Instrumenting the effect in the same production bundle over 12 loads gives
effectRuns=[1] and cancels=[0,1]: `targetDocument` is identity-stable and the
effect never re-runs, so the outcome is decided purely by the flush ordering. The
cross-tab is one-to-one -- rafIdRef.current null at the single effect run -> no
cancel -> 30px (1 load); "set" -> cancel -> 12px (11 loads).
Completeness, four arms x 12 loads on a dev bundle with page-level instrumentation
of ResizeObserver, requestAnimationFrame and cancelAnimationFrame:
arm token effect cleanup roCtor obs disc raf s/c/fired
unpatched / plain / dev 0/12 1 0 1 1 0 1/1/0
unpatched / StrictMode / dev 12/12 2 0 2 2 1 2/1/1
cleanupFix / plain / dev 12/12 1 0 1 1 0 1/0/1
cleanupFix / StrictMode / dev 12/12 2 1 2 2 1 2/1/1
No observer leak (observes == constructs in every arm), no double-fire (the aside
frame fires exactly once wherever it fires at all), and no StrictMode regression.
The fix also strictly improves unmount: today nothing cancels the pending frame,
which fires against a detached ref and is swallowed by optional chaining.
Note for anyone reproducing this: it must be a PRODUCTION build with no
StrictMode. Under StrictMode in a dev build the defect is invisible -- the
double-invoke detaches and re-attaches the ref, the observer fires a second time
after the effect has already run, and the write lands 12 of 12. A StrictMode dev
Storybook will not show it.
Regression coverage, both verified to fail without this change: TagPickerControl
now has two tests that pin the ordering deterministically. The observer is
attached by a ref callback, so `observe()` runs in the commit phase, before React
flushes the passive effect; a ResizeObserver stub whose `observe()` invokes its
callback synchronously therefore puts a frame in flight by the time the effect
runs -- exactly the state the production race lands on -- without depending on
real timing, which jsdom cannot reproduce. The end-to-end probes that produced the
numbers above are kept as the timing-level gate at
.scratch/windmod-loop/revalidation/tag-picker-tpa-{determinism,effect-count,lifecycle}.mjs.
Verified: react-tag-picker build, type-check and lint pass; its suite is 1 failed /
181 passed both with and without this change (pre-existing @fluentui/react-icons
snapshot drift -- SVG path data and the `fui-Icon` class -- unrelated to it), the
two new tests included in the 181.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013wmpBCYJpDJCLXcScCWz1i
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Aj9uA3rCVgosnh2zNn8qkc
Dmytro Kirpa (dmytrokirpa)
requested review from
Bernardo Sunderhus (bsunderhus) and
Victor Genaev (mainframev)
and
a balanced review from Copilot
and removed request for
Victor Genaev (mainframev)
September 1, 2026 10:25
Dmytro Kirpa (dmytrokirpa)
assigned
Paul Mardling (PaulGMardling) and unassigned
Paul Mardling (PaulGMardling)
Dmytro Kirpa (dmytrokirpa)
requested a review
from Paul Mardling (PaulGMardling)
September 1, 2026 10:26
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Dmytro Kirpa (dmytrokirpa)
September 1, 2026 10:26
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Pull request overview
Fixes a TagPickerControl race by deferring animation-frame cancellation to effect cleanup.
Changes:
- Preserves the mount-time aside-width update and cancels pending work on unmount.
- Adds deterministic regression tests.
- Adds a patch change file.
Merge confidence: 100/100
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
useTagPickerControl.tsx |
Moves frame cancellation into cleanup. |
TagPickerControl.test.tsx |
Tests mount-time writing and unmount cancellation. |
| Change file | Records the patch release. |
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In
useTagPickerControl, aResizeObservercallback schedules the only write of--fui-TagPickerControl-aside-widthinside arequestAnimationFrame, but the matchingcancelAnimationFrameruns in the effect body rather than the effect's cleanup. The observer's first callback fires from a ref callback in the commit phase — before React flushes the passive effect — so the effect body frequently cancels the very frame that carries the write. Measured on a production build with StrictMode off: the token was written on 3 of 12 fresh page loads, an 18px resting-width swing between runs of the same page. (Under StrictMode in a dev build the double-invoke masks the race completely, which is why it survives Storybook probing.)The fix moves the cancel into the effect's cleanup: 12/12 writes after, no observer leak, no double-fire, no StrictMode regression, and a strictly better unmount (today nothing cancels a pending frame at unmount). Two regression tests are added that pin the commit-phase-before-effect ordering deterministically; both were verified to fail without the fix.
Fixes #36649.
Extracted from #36656 per maintainer request — each in-tree fix from that PR as an isolated change.