Fix react-19 external anchor replacement in AnchoredOverlay and share Vitest setup

[Copilot]

Changelog

New

• Added a shared @primer/vitest-config workspace package for the common Vitest console-enforcement setup.
• Added package-local config/vitest/setup.js entrypoints so each package can consume the shared Vitest setup without relying on a top-level setup script path.

Changed

• Updated AnchoredOverlay to re-sync CSS anchor-positioning state when an external anchorRef DOM node is replaced while the overlay remains open.
• Preserved existing internal renderAnchor behavior while fixing the React 19 external-anchor remount case that caused anchor-name and position-anchor to fall out of sync.
• Added an inline code comment documenting the React 19-specific external anchor replacement behavior.
• Updated package Vitest configs to use the new shared setup package through package-local setup entrypoints.

Removed

• Removed the top-level shared Vitest setup script at script/vitest/setup.ts.

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

• Analyzed the failing GitHub Actions test (react-19) job logs for run 25577112553, job 75087602292.
• Reproduced the original failure locally against packages/react/src/ActionMenu/ActionMenu.test.tsx.
• Verified the AnchoredOverlay fix with:
  • npx vitest --config vitest.config.browser.mts run src/AnchoredOverlay/AnchoredOverlay.test.tsx
  • npx vitest --config vitest.config.browser.mts run src/ActionMenu/ActionMenu.test.tsx
• Verified the shared Vitest setup refactor with:
  • npm run lint -- --no-cache
  • npm run type-check
  • cd packages/styled-react && npx vitest --run src/__tests__/exports.test.ts
  • cd packages/react && npx vitest --config vitest.config.browser.mts run src/AnchoredOverlay/AnchoredOverlay.test.tsx
• Ran secret scanning and final automated review / CodeQL validation.

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]

⚠️ No Changeset found

Latest commit: 9ebb459

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[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.

[joshblack]

@copilot fix CI, it is currently broken. Don't submit changes for review until CI is green

[Copilot]

@copilot fix CI, it is currently broken. Don't submit changes for review until CI is green

Fixed in b0e2fcd. Local validation is green (CI=true npm run test, npm run lint -- --no-cache, npm run type-check, npm run build, npm run lint:css, npm run format:diff -- --cache=false). The new GitHub Actions runs are currently action_required with no jobs, so they need approval before CI can turn green; I have not submitted the PR for review.

[joshblack]

@copilot create a shared vitest config package that can be used instead of a common script at the top-level

[Copilot]

@copilot create a shared vitest config package that can be used instead of a common script at the top-level

Addressed in 9ebb459. Added a shared @primer/vitest-config workspace package for the common Vitest setup and updated the package-level Vitest configs to use local setup entrypoints that import it.