AnchoredOverlay: Disable CSS Anchor Positioning if fallbacks is not supported

[TylerJDev]

Disables CSS anchor positioning if position-try-fallbacks or positionVisibility are not supported. This will only disable CSS anchor positioning for a small number of browsers, but will ensure that all users have a level of support, either through CSS anchor positioning or the polyfill.

Changelog

Changed

• Disable CSS anchor positioning if certain CSS rules are not supported on the current browser.

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

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 (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 467c12a

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

Not sure what this means? Click here to learn what changesets are.

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. 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]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/21756

[Copilot]

Pull request overview

Disables the native CSS anchor-positioning path in AnchoredOverlay unless the browser supports additional related CSS features, ensuring the JS/polyfill positioning path is used in partially-supported browsers.

Changes:

• Tightened client-side feature detection for native CSS anchor positioning to require position-try-fallbacks and position-visibility support.
• Added a patch changeset entry documenting the behavioral change.

Show a summary per file

File	Description
packages/react/src/AnchoredOverlay/AnchoredOverlay.tsx	Makes native CSS anchor-positioning opt-in only when additional related CSS features are detected.
.changeset/crazy-kings-sip.md	Adds a patch changeset note for the AnchoredOverlay behavior change.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 2

[primer-integration]

Integration test results from github/github-ui:

CI Passed
VRT Passed
Projects Passed

All checks passed!