Fix PageLayout whitespace gap on narrow viewports

[liuliu-dev]

Fixes a whitespace gap that appears on narrow viewports when PageLayout has a forced height (e.g., height: 100%).

Closes https://github.com/github/primer/issues/6253

Problem

When PageLayout has a fixed/forced height and the viewport is narrow, the PageLayoutContent container becomes taller than its children need. With flex-direction: row (default) + flex-wrap: wrap, the extra vertical space causes a visible whitespace gap above the pane.

Solution

Add flex-direction: column to .PageLayoutContent on narrow viewports.

Testing

Added a regression test story FixedHeightResponsive that reproduces the issue.

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]

🦋 Changeset detected

Latest commit: 0e72d0d

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]

👋 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 fixes a whitespace gap that appears in PageLayout on narrow viewports when the component has a forced height (e.g., height: 100%). The gap occurs because flex-direction: row with flex-wrap: wrap distributes extra vertical space as gaps between wrapped rows.

Key Changes:

• Added flex-direction: column to .PageLayoutContent on narrow viewports to prevent unwanted wrapping behavior
• Added comprehensive regression test story FixedHeightResponsive demonstrating the fix

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/react/src/PageLayout/PageLayout.module.css	Added flex-direction: column media query for narrow viewports to fix whitespace gap
packages/react/src/PageLayout/PageLayout.responsive.stories.tsx	Added FixedHeightResponsive regression test story with detailed documentation
.changeset/cyan-tigers-reply.md	Added changeset for patch release

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/10330

[primer-integration]

🔬 github-ui Integration Test Results

Check	Status	Details
CI	✅ Passed	View run
Projects (Memex)	✅ Passed	View run
VRT	✅ Passed	View run

✨ All checks passed! Your integration PR is ready for review.