UnderlineNav Overflow Fix

[TylerJDev]

Part of https://github.com/github/primer/issues/6291

Improves CLS by adding overflow: hidden during calculation of menu items.

Base PR: #7463

Changelog

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• 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: 7033319

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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.

[francinelucca]

nit: can we do data-ready={ready ? '' : undefined}? I swear we have docs about this somewhere but I can't find 😅

[TylerJDev]

For sure! I just added undefined instead

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/12629

[primer-integration]

Integration test results from github/github-ui:

  CI Failed
  VRT Passed
  Projects Passed

CI check runs linting, type checking, and unit tests. Check the workflow logs for specific failures.

Need help? If you believe this failure is unrelated to your changes, please reach out to the Primer team for assistance.