perf: derive Autocomplete re-sort and LabelGroup truncation during render

[Copilot]

Standalone performance refactor (originally split out of #8001). It narrows two mixed-responsibility effects so derivable state updates happen during render, while genuine side effects stay in effects. Behavior-preserving: no API or visual changes.

Changelog

New

• None.

Changed

• AutocompleteMenu

  • Close-time re-sorting is now derived during render instead of running inside the onOpenChange effect.
  • The comparator runs only while the menu is closed (showMenu === false), so it never runs on the frequent renders that happen while the menu is open. AutocompleteOverlay remounts this component on each open/close, so a render-time previous-value transition can't be observed here — guarding on the closed state is what replaces the old close-time effect.
  • onOpenChange remains in useEffect as the actual side effect.

• LabelGroup

  • Numeric visibleChildCount truncation now derives visibilityMap from child indices during render instead of reading committed DOM state in an effect.
  • 'auto' truncation remains effect-driven via IntersectionObserver.
  • Added a shallow-equality guard and a memoized derived map to avoid redundant render-phase updates.

• Tests

  • Tightened Autocomplete coverage to assert onOpenChange fires correctly across open/close transitions.
  • Added LabelGroup rerender coverage for numeric truncation updates.

Close-time sort, derived during render while the menu is closed:

if (showMenu === false) {
  const itemIdSortResult = [...sortedItemIds].sort(/* ... */)
  const sortResultMatchesState =
    itemIdSortResult.length === sortedItemIds.length &&
    itemIdSortResult.every((element, index) => element === sortedItemIds[index])

  if (!sortResultMatchesState) {
    setSortedItemIds(itemIdSortResult)
  }
}

Removed

• The redundant effect-driven numeric-truncation update path in LabelGroup.
• The now-unneeded set-state-in-effect / derived-state lint suppressions around Autocomplete close-time sorting.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; behavior-preserving internal refactor only, with no API or visual change.

Testing & Reviewing

Review by change site:

• Autocomplete — confirm close-time sorting still happens only while the menu is closed, and that onOpenChange still fires with the current open state and remains effect-driven.
• LabelGroup — confirm numeric visibleChildCount truncation updates correctly across rerenders, and that 'auto' truncation, inline expand/collapse, and overlay overflow behavior are unchanged.

Validated with:

• npx vitest run Autocomplete/Autocomplete.test LabelGroup/LabelGroup.test (40 passing)
• npx eslint packages/react/src/Autocomplete/AutocompleteMenu.tsx packages/react/src/LabelGroup/LabelGroup.tsx packages/react/src/Autocomplete/Autocomplete.test.tsx packages/react/src/LabelGroup/LabelGroup.test.tsx

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 4cecd51

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

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[Copilot]

Pull request overview

This PR is a follow-up refactor to #8001 that narrows two previously “splittable later” effect sites by deriving state during render for derivable transitions (while keeping real side effects in effects). The focus is Autocomplete close-time reordering and LabelGroup numeric truncation.

Changes:

• AutocompleteMenu: Track showMenu transitions during render and trigger close-time re-sorting on the open→closed transition; keep onOpenChange in an effect.
• LabelGroup: For numeric visibleChildCount, derive the truncation visibilityMap from child indices (not committed DOM) and add a shallow equality guard; keep 'auto' truncation effect-driven via IntersectionObserver.
• Tests: Strengthen coverage for onOpenChange(false → true → false) and add rerender coverage for numeric LabelGroup truncation updates.

Show a summary per file

File	Description
packages/react/src/LabelGroup/LabelGroup.tsx	Derives numeric truncation visibility from child indices and updates render path to use a cached childArray.
packages/react/src/LabelGroup/LabelGroup.test.tsx	Adds rerender assertions to ensure numeric truncation updates correctly when visibleChildCount changes.
packages/react/src/Autocomplete/AutocompleteMenu.tsx	Moves close-time sorting to a render-time transition check while keeping onOpenChange effect-driven.
packages/react/src/Autocomplete/Autocomplete.test.tsx	Tightens onOpenChange assertions to validate open/close sequencing.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 2

[mattcosta7]

Rescoped this to a standalone PR: retargeted the base from copilot/derive-state-from-effects to main and dropped the old #8001 leftovers (useMedia, SelectPanel, ToggleSwitch, ValidationAnimationContainer, Dialog, TreeView, SelectPanel2), which are handled by their own focused PRs. It now contains only the Autocomplete close-time re-sort and LabelGroup numeric-truncation render derivations (+ tests). This also clears the merge conflict that appeared when #8001 was rescoped.