Defer focusZone initialization to avoid blocking paint

[LisaKr]

Closes #

focusZone() synchronously walks the entire container DOM and sets tabindex="-1" on all focusable elements when it initializes. For a list with 25 rows (e.g. GitHub issues list), this takes ~18ms. The tabindex mutations also trigger a React re-render of the entire list (~48ms total blocked before paint).

This PR wraps the beginFocusManagement() call inside useFocusZone in requestAnimationFrame so it runs after the browser paints instead of blocking the initial render.

Why this is safe:

• Setting tabindex is invisible — it doesn't change anything the user can see
• Keyboard navigation activating one frame later (~16ms) is imperceptible since users can't press keys before they see the page
• The AbortController cleanup still works via cancelAnimationFrame + abort()

Changelog

Changed

• useFocusZone now defers focusZone() initialization to a requestAnimationFrame callback to avoid blocking paint on large lists

New

Removed

Rollout strategy

• Patch release

Testing & Reviewing

• TreeView tests updated to flush the deferred RAF via vi.advanceTimersByTime(16) after render (tests already use vi.useFakeTimers)
• ActionList and ActionBar tests pass without changes
• All 51 TreeView tests pass, all 15 ActionList tests pass

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

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Latest commit: 1e85347

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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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