remove primer_react_overlay_overflow FF from the codebase

[francinelucca]

Relates to https://github.com/github/feature-flag-lifecycle/issues/5979

We ended up never rolling out this feature flag (introduced here #5129), so I'm cleaning it up.

Changelog

Removed

• all code references to primer_react_overlay_overflow FF

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 (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 2330a0b

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-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.

[Copilot]

Pull request overview

This PR removes the primer_react_overlay_overflow feature flag from the codebase. The feature flag was introduced in PR #5129 to enable responsive reflow behavior for the Overlay component, but was never rolled out to production. This is a cleanup operation that removes all references to the unused feature flag while preserving the existing default behavior.

Changes:

• Removed the useFeatureFlag import and feature flag check from Overlay component
• Simplified the data-reflow-container attribute logic to only depend on the preventOverflow prop
• Removed the feature flag-specific test case
• Removed the feature flag definition from DefaultFeatureFlags

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/react/src/Overlay/Overlay.tsx	Removed useFeatureFlag import and simplified data-reflow-container logic to only check preventOverflow prop, maintaining the same behavior since the feature flag was never enabled
packages/react/src/Overlay/Overlay.test.tsx	Removed test case that verified behavior when feature flag was enabled
packages/react/src/FeatureFlags/DefaultFeatureFlags.ts	Removed primer_react_overlay_overflow feature flag definition

[TylerJDev]

Thank you ✨!

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/12055

[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.