UnderlineNav: Prevent borders from triggering overflow menu

[ShiyunXu]

Closes N/A

Prevents consumer border styles on the wrap-mode UnderlineNav wrapper from changing its vertical box model and incorrectly displaying the overflow menu. This supersedes the stale approach from #8165 and addresses the integration issue seen in github/github-ui#26710.

Changelog

New

None.

Changed

• UnderlineNav now resets borders on every side while using wrap-mode overflow detection.

Removed

None.

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

Verify custom borders on UnderlineNav no longer cause the More menu to appear when all items fit.

• UnderlineNav browser tests (22 tests)
• Stylelint and formatting checks

[changeset-bot]

🦋 Changeset detected

Latest commit: 744f5c7

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]

⚠️ 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.

[Copilot]

Pull request overview

This PR hardens UnderlineNav wrap-mode overflow detection by preventing consumer-applied borders on the wrapper from altering the wrapper’s box model and falsely triggering the overflow (“More”) menu.

Changes:

• Reset wrapper borders on all sides when UnderlineNav is in wrap overflow mode.
• Add a patch changeset describing the behavior fix for @primer/react.

Show a summary per file

File	Description
packages/react/src/internal/components/UnderlineTabbedInterface.module.css	Updates wrap-mode wrapper styles to force borders to 0 so external border overrides can’t affect overflow detection.
.changeset/underline-wrapper-force-no-border.md	Adds a patch changeset documenting the UnderlineNav overflow-menu fix.

Review details

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• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

[primer-integration]

Integration test results from github/github-ui PR:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!