[CSS Anchor Positioning] Disable feature flag if portalContainerName is true

[TylerJDev]

Closes https://github.com/github/primer/issues/6648

Disables primer_react_css_anchor_positioning if portalContainerName is true.

Changelog

Changed

• Disables primer_react_css_anchor_positioning if portalContainerName is present.

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: 301953d

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/19933

[primer-integration]

Integration test results from github/github-ui:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!

[Copilot]

Pull request overview

This PR narrows when AnchoredOverlay uses CSS anchor positioning so overlays fall back to JS positioning when a named portal container is provided. It targets the overlay/portal integration in @primer/react, specifically the feature-flagged CSS anchor positioning path.

Changes:

• Gate cssAnchorPositioning behind the absence of overlayProps.portalContainerName.
• Preserve existing popover/CSS-anchor behavior when no named portal container is supplied.
• Add a patch changeset describing the AnchoredOverlay behavior change.

Show a summary per file

File	Description
packages/react/src/AnchoredOverlay/AnchoredOverlay.tsx	Disables the CSS anchor positioning code path when overlayProps.portalContainerName is set.
.changeset/evil-coins-tease.md	Records the AnchoredOverlay behavior change for release notes.

Copilot's findings

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

[llastflowers]

💖