resize-improvemet

[mattcosta7]

related to https://github.com/github/pull-requests/issues/21237
Closes #

Changelog

New

Changed

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

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 (Learn more about how to run integration tests)

[changeset-bot]

⚠️ No Changeset found

Latest commit: 6914dd2

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[github-actions]

👋 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. Or, apply the integration-tests: skipped manually label to skip these checks.

[Copilot]

Pull request overview

This PR optimizes the PageLayout pane resize handling by implementing a two-stage approach: throttled CSS variable updates (16ms) for immediate visual feedback during resize, and debounced synchronization (150ms) of refs, ARIA attributes, and React state when resizing stops. The changes improve performance by reducing expensive operations (getComputedStyle calls, React re-renders) while maintaining visual responsiveness.

Key Changes

• Separated CSS updates from state synchronization for better resize performance
• Added requestAnimationFrame-based throttling for smooth 60fps CSS updates
• Used React 18's startTransition for non-urgent state updates (maxPaneWidth)
• Added comprehensive test coverage for the new throttle/debounce logic

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
packages/react/src/PageLayout/usePaneWidth.ts	Refactored resize handler to use separate updateCSSOnly (throttled) and syncAll (debounced) functions; added startTransition for maxPaneWidth updates; improved performance by limiting getComputedStyle calls to breakpoint crossings
packages/react/src/PageLayout/usePaneWidth.test.ts	Added comprehensive tests for throttled CSS updates, debounced ARIA/state sync, viewport clamping, and cleanup behavior; reorganized existing resize listener tests for clarity
packages/react/src/PageLayout/PageLayout.tsx	Added safety clamp to handle edge case where user starts dragging before debounced resize completes
packages/react/src/PageLayout/PageLayout.module.css	Added documentation comment explaining that --pane-max-width CSS variable is managed by JavaScript

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/8744

[primer-integration]

🔬 github-ui Integration Test Results

Check	Status	Details
CI	✅ Passed	View run
Projects (Memex)	⏳ Pending	Waiting for workflow to complete
VRT	✅ Passed	View run