Extract reusable "descendant registry" pattern from ActionBar, with new support for reordering elements

[Copilot]

Adds unit tests for createDescendantRegistry and fixes a bug where pure item reordering (no mounts/unmounts/value changes) failed to update the registry order.

Changelog

New

• Unit tests for createDescendantRegistry covering: descendants inside wrapper components, React fragments, mid-list insertion, unmounting, reordering via key, and deep-tree prepending

Changed

• createDescendantRegistry Provider now detects when children reference changes without any descendant lifecycle events (the reorder case) and triggers a full rebuild, so the registry always reflects the current render order

Root cause of the reorder bug: When items are reordered using React key props (no remount), no child effects re-run because no hook dependencies change. The commitWorkingRegistry effect was a no-op in the idle state, so the registry stayed stale.

Fix: Track prevChildrenRef in commitWorkingRegistry. When children changes but no descendant fired a register/unregister (i.e., workingRegistryRef is still 'idle'), queue a rebuild — this increments key in context, causing all children to re-run their effects in the new DOM order.

useEffect(function commitWorkingRegistry() {
  const childrenChanged = prevChildrenRef.current !== children
  prevChildrenRef.current = children

  if (workingRegistryRef.current instanceof Map) {
    setRegistry(workingRegistryRef.current)
    workingRegistryRef.current = 'idle'
  } else if (workingRegistryRef.current === 'idle' && childrenChanged) {
    // Reorder detected — rebuild so items re-register in new render order
    workingRegistryRef.current = 'queued'
    setKey(prev => prev + 1)
  }
})

Removed

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Six new tests in packages/react/src/utils/__tests__/descendant-registry.test.tsx. Each test creates an isolated registry via a createTestRegistry() factory to prevent cross-test context leakage.

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)

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Latest commit: 628c90e

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