chore(Overlay): Remove the CSS modules feature flag from the Overlay component

[jonrohan]

Closes https://github.com/github/primer/issues/4135

Changelog

New

Changed

Removed

Remove the CSS modules feature flag from the Overlay component

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

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: cb2d313

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	Minor

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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	99.37 KB (-0.38% 🔽)
packages/react/dist/browser.umd.js	99.65 KB (-0.24% 🔽)

[primer-integration]

👋 Hi from github/github! Your integration PR is ready: https://github.com/github/github/pull/375243

[primer-integration]

🟢 golden-jobs completed with status success.

[Copilot]

Pull Request Overview

This PR removes the CSS modules feature flag from the Overlay component to simplify and streamline the component’s styling logic. Key changes include:

• Updating the snapshot for AnchoredOverlay by removing the CSS modules based styling.
• Refactoring the Overlay component to use BoxWithFallback instead of a styled component based on a feature flag.
• Skipping two alignment tests for the Overlay component.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/react/src/tests/snapshots/AnchoredOverlay.test.tsx.snap	Updated snapshot reflecting removal of the old CSS modules styles
packages/react/src/Overlay/Overlay.tsx	Removed the CSS modules feature flag and refactored the component to use BoxWithFallback
packages/react/src/Overlay/Overlay.test.tsx	Marked two alignment tests as skipped
.changeset/modern-doodles-learn.md	Updated changeset documenting the removal

Comments suppressed due to low confidence (2)

packages/react/src/Overlay/Overlay.test.tsx:205

• Consider adding an inline comment explaining why this alignment test is skipped and under what conditions it should be re-enabled.

it.skip('should right align when given `right: 0` and `position: fixed`', async () => {

packages/react/src/Overlay/Overlay.test.tsx:235

• Consider adding an inline comment that clarifies the rationale behind skipping this alignment test to improve maintainability and clarity for future developers.

it.skip('should left align when not given position and left props', async () => {