perf: add contain:layout to high-churn overflow containers

[mattcosta7]

Closes #

Applies CSS contain: layout to a handful of high-churn overflow containers so their frequent, resize/observer-driven reflows are isolated to their own subtree and can no longer invalidate ancestor layout. This mirrors the existing contain: layout style paint pattern already used by PageLayout during drag/resize.

Changelog

Changed

• ActionBar (.List + .OverflowContainer): scroll-timeline overflow detection + JS visibility toggling are now contained.
• UnderlineNav / UnderlinePanels (shared .UnderlineItemList): ResizeObserver-driven item swapping into the overflow menu is now contained.
• LabelGroup (.Container): IntersectionObserver-driven show/hide of overflowing labels is now contained.

All targets already set overflow: hidden and their overlays/menus are portaled or rendered separately, so the new containing block/stacking context has no visual effect. contain: paint was intentionally not used (no clipping), and contain: size was avoided so elements still size to content.

New

None

Removed

None

Rollout strategy

• Patch release

Testing & Reviewing

Pure CSS containment change. Unit tests for ActionBar, UnderlineNav, and LabelGroup pass. Because containment establishes new stacking/containing-block contexts, this should get a visual regression (@vrt) pass on ActionBar, UnderlineNav, UnderlinePanels, and LabelGroup — verifying overflow menus and expanded label overlays still render outside their containers.

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

[changeset-bot]

🦋 Changeset detected

Latest commit: 39ac2c6

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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Click here if you're a maintainer who wants to add another 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. If this doesn't work, you can also use the original workflow here. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

To publish a canary release for integration testing, apply the Canary Release label to this PR.

[primer-integration]

Integration test results from github/github-ui PR:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!

[Copilot]

Pull request overview

Applies CSS contain: layout to a few high-churn, overflow-related containers in @primer/react to isolate frequent reflows (ResizeObserver/IntersectionObserver/overflow-driven UI) and reduce ancestor layout invalidation.

Changes:

• Add contain: layout to ActionBar list + overflow container to contain wrap/measure churn.
• Add contain: layout to the shared underline tab item list used by UnderlineNav and UnderlinePanels.
• Add contain: layout to LabelGroup container to contain IntersectionObserver-driven show/hide churn, plus a patch changeset.

Show a summary per file

File	Description
packages/react/src/ActionBar/ActionBar.module.css	Adds layout containment to overflow-detection/wrapping containers to isolate reflows.
packages/react/src/internal/components/UnderlineTabbedInterface.module.css	Adds layout containment to the underline tab list to isolate ResizeObserver-driven layout churn.
packages/react/src/LabelGroup/LabelGroup.module.css	Adds layout containment to the label row to isolate IntersectionObserver-driven visibility changes.
.changeset/contain-layout-high-churn.md	Patch changeset documenting the containment update across components.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Low