Overlay: Fix position for fullscreen overlay on narrow screens

[siddharthkp]

Note

Current status: main is failing integration tests, so need to run integration tests again once that is cleared up

• Bug introduced in AnchoredOverlay: Add responsive variant prop #5759
• More context on slack
• Used in SelectPanel under a feature flag

Full screen overlay should have position: fixed (not absolute) so that it renders at top: 0 even when the page has scrolled.

(would be nice to add a vrt test, but i'm in a rush 😅)

Rollout strategy

• Patch release (and feature flagged)
• 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: 382191d

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 depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	105.91 KB (-0.01% 🔽)
packages/react/dist/browser.umd.js	106.24 KB (+0.05% 🔺)

[primer-integration]

👋 Hi from github/github! Your integration PR is ready: https://github.com/github/github/pull/370328

[primer-integration]

🟢 golden-jobs completed with status success.