fixed react 19 errors

[rezrah]

Summary

Fixes major bug with React 19 release candidate, which is causing errors in dotcom when using React Next FF.

Makes parts of react-dom full externals of the library, to prevent conflict in React 19+/

What should reviewers focus on?

• Check our Storybook, which uses React 18 still works okay
• Check integration tests pass (currently use React 18)

Steps to test:

1. Storybook

Contributor checklist:

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• New visual snapshots have been generated / updated for any UI changes
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

Screenshots:

Fixed and tested in Next.js RC

Before	After

[changeset-bot]

🦋 Changeset detected

Latest commit: 49ae5c0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@primer/react-brand	Minor
@primer/brand-primitives	Minor
@primer/brand-e2e	Minor
@primer/brand-fonts	Minor
@primer/brand-config	Minor
@primer/brand-storybook	Minor

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]

🟢 No design token changes found

[github-actions]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.

[joshfarrant]

👍

Do you think it's worth bumping our Next.js integration test to use the Next 15 RC so we can keep an eye on React 19 compatibility in the pipelines? Once Next 15 reaches GA we can revert so we're not constantly tracking an RC.

[rezrah]

👍

Do you think it's worth bumping our Next.js integration test to use the Next 15 RC so we can keep an eye on React 19 compatibility in the pipelines? Once Next 15 reaches GA we can revert so we're not constantly tracking an RC.

The current Next.js test is still valid, and arguably more important than the RC as we need to ensure compat with latest stable Next.js. We could create a second integration test though specifically for React 19, and use Next.js RC as the mechanism to verify. That's a good shout, I'll take a look later this week.