Fix PageLayout pane layout shift when content loads asynchronously

[jonrohan]

Closes https://github.com/github/pull-requests/issues/23557

Before	After

Overview

Fixes a subtle layout shift on pages using PageLayout.Pane where the sidebar content is loaded asynchronously. When the pane content rendered after the initial load, the page would visibly jump because the PaneWrapper element had width: auto and didn't reserve space for the pane.

Root cause: The pane width CSS custom properties (--pane-width-size, --pane-min-width, --pane-max-width, etc.) were set on the inner pane element rather than the outer PaneWrapper. This meant the wrapper couldn't use them to reserve the correct width before pane content loaded.

Fix:

• Moved the pane width CSS custom properties from the inner pane div up to the PaneWrapper element
• Changed PaneWrapper from width: auto to width: var(--pane-width-size) so it reserves the correct pane width immediately, preventing layout reflow when content loads in

Changelog

New

Changed

• PageLayout.Pane: Pane width CSS variables are now set on the PaneWrapper element instead of the inner pane div, and the wrapper width uses var(--pane-width-size) instead of auto

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

1. Use a PageLayout with a Pane that loads its content asynchronously (e.g., via useEffect or data fetching)
2. Observe that the page no longer shifts/jumps when the pane content appears
3. Verify that pane widths (small, medium, large, and custom) still render correctly

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]

⚠️ No Changeset found

Latest commit: c4ad237

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]

⚠️ 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.

[github-actions]

Uh oh! @jonrohan, at least one image you shared is missing helpful alt text. Check your pull request body to fix the following violations:

• Images should have meaningful alternative text (alt text) at line 9
• Images should have meaningful alternative text (alt text) at line 9

Alt text is an invisible description that helps screen readers describe images to blind or low-vision users. If you are using markdown to display images, add your alt text inside the brackets of the markdown image.

Learn more about alt text at Basic writing and formatting syntax: images on GitHub Docs.

🤖 Beep boop! This comment was added automatically by github/accessibility-alt-text-bot.