Add AnchoredOverlay outside-top content growth regression fixture

[siddharthkp]

Closes #

This PR adds a fixture story and VRT test to reproduce and guard against a visual regression where AnchoredOverlay with side="outside-top" and async-loading content briefly flickers downward before repositioning upward.

The bug:
When content loads asynchronously inside an outside-top anchored overlay:

1. Content loads → overlay height increases naturally (downward)
2. Position recalculates → overlay jumps to correct outside-top position
3. This causes a visible flicker

This PR:

• Adds RepositionAfterContentGrowsWithOutsideTop story to AnchoredOverlay.dev.stories.tsx
• VRT test runs automatically via existing AnchoredOverlay test matrix (with and without CSS anchor positioning flag)
• Does not include a fix yet — this is a regression fixture only

Note: With CSS anchor positioning (primer_react_css_anchor_positioning flag enabled), the browser handles positioning natively via bottom: anchor(top) which causes content to grow upward naturally — no flicker. The bug only affects the JS positioning fallback path.

Changelog

New

• AnchoredOverlay.dev.stories.tsx: Added RepositionAfterContentGrowsWithOutsideTop story

Changed

• None

Removed

• None

Rollout strategy

• None; this is a fixture/test-only change with no runtime impact

Testing & Reviewing

1. Open Storybook → AnchoredOverlay → Dev → Reposition After Content Grows With Outside Top
2. Click the button to open the overlay
3. Wait 2 seconds for content to load
4. Observe the brief downward flicker before the overlay repositions upward (JS path)
5. With primer_react_css_anchor_positioning: true globals, the flicker should not occur

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]

⚠️ No Changeset found

Latest commit: 9dabdb5

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a 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. 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]

🤖 Lint issues have been automatically fixed and committed to this PR.