Upgrade to React v19

[rezrah]

Summary

Towards https://github.com/github/primer/issues/6134

Improves support for React 19, which is now the default React version for all monorepo packages except Docs.

List of notable changes:

• Improved ref as props support for multiple components (see changeset), which are treated differently in v19
• Monorepo now uses (and ships) @types/react-*@v19 by default
• Documentation site downgraded to 18 and excluded as a workspace package as it has a dependency on both @primer/react and @primer/doctocat-nextjs to support v19 first 🔁
• Unwound many instances of forced type assertions in favour of type narrowing predicates and parameterized generics
• Fixed a race condition in our AXE tests whereby page container was being reassigned too early. While this also happened before this PR, the workaround of restarting the workflow no longer works.

ℹ️ There should not be any functional or presentational changes in this PR. Changes should also remain backwards compatible with React@v18.

Any drops in test coverage are due to additional LoC for null ref support. These can be reviewed in follow-up PR's individually. We have sufficient test coverage for ref in most components now.

What should reviewers focus on?

• Verify that no visual changes have occured (0 snapshot diffs)

Steps to test:

1. Verify that no visual changes have occured (0 snapshot diffs / No visual changes comment in this PR)
2. Go through Storybook preview and check all examples are loading correctly
3. Go through Docs preview and check all examples are loading correctly

Supporting resources (related issues, external links, etc):

• Tracking issue
• Platform (Web Systems) tracking issue

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
• UI Changes contain new visual snapshots (generated by adding update snapshots label to the PR)
• 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:

N/A 🤞

[changeset-bot]

🦋 Changeset detected

Latest commit: a8a497c

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

This PR includes changesets to release 7 packages

Name	Type
@primer/react-brand	Minor
@primer/brand-css	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

[Copilot]

Pull request overview

This PR upgrades the Primer Brand monorepo to React v19 as the default version while maintaining backwards compatibility with React v18. The primary changes involve improved type safety for React elements, better ref handling, and removal of deprecated patterns.

Key changes:

• Upgraded React and React types to v19 across main packages (react, e2e, storybook, design-tokens, css)
• Improved type assertions on React.isValidElement and React.cloneElement calls throughout component tree
• Enhanced ref forwarding patterns to comply with React 19's stricter type requirements
• Replaced react-is Fragment detection with custom isFragmentElement utility
• Updated inert attribute handling in RiverAccordion (now boolean instead of string)
• Configured separate tsconfig files for tests and builds with appropriate type inclusions
• Excluded next-docs from workspace (downgraded to React 18 due to dependencies)

Reviewed changes

Copilot reviewed 77 out of 80 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/react/package.json	Updated React types to v19, added @types/chai and @types/node dependencies
packages/react/tsconfig.json	Added dom.iterable lib and explicit types array for improved type resolution
packages/react/tsconfig.test.json	New test-specific config with jest types included
packages/react/tsconfig.build.json	Expanded build exclusions to cover all test file patterns
packages/react/src/react-jsx.d.ts	New JSX namespace declaration for React 19 runtime compatibility
packages/react/src/utils/isFragmentElement.ts	Custom Fragment type guard replacing react-is dependency
packages/react/src/river/RiverAccordion/RiverAccordion.tsx	Updated inert attribute to boolean with proper type assertion
packages/react/src/river/River/River.tsx	Moved ref prop before other props, added type parameters to isValidElement
packages/react/src/hooks/*.ts	Enhanced ref type signatures to explicitly include null
packages/react/src/forms/*.tsx	Improved type safety with explicit component prop types
packages/react/src//.tsx	Systematic improvements to React.isValidElement and cloneElement type parameters across 40+ components
packages/react/jest.config.js	Added ts-jest transform with tsconfig.test.json for proper test type resolution
packages/e2e/, packages/design-tokens/, packages/css/**	Added @types/node and updated tsconfig to include node types
package.json	Upgraded React to 19.2.0, added postinstall hook for next-docs, excluded next-docs from workspace
apps/storybook/package.json	Updated React and React types to v19
apps/next-docs/package.json	Pinned to React 18 types with overrides for compatibility
.changeset/hungry-readers-switch.md	Documented breaking and non-breaking changes

[github-actions]

🟢 No visual differences found

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

[github-actions]

🟢 Unit test coverage changes found

Unit test coverage has been updated through this PR.

Changes: 0 new tests, 0 removed tests, 6 improved, 13 decreased

Component/Hook	Statements	Functions	Branches	Change
ButtonGroup	100.0% 88.9%	100.0%	100.0%	-11.1%
VideoPlayer	100.0% 95.0%	100.0%	85.7% 84.5%	-5.0%
Footnotes	100.0% 95.2%	100.0%	100.0%	-4.8%
FAQ	94.2% 90.2%	100.0%	92.0% 90.6%	-4.0%
Card	100.0% 96.6%	100.0%	91.4% 84.6%	-6.8%
BreakoutBanner	97.7% 95.5%	100.0%	89.8% 89.6%	-2.2%
Accordion	97.9% 100.0%	100.0%	94.1% 100.0%	+5.9%
Bento	82.4% 80.5%	94.4%	74.8%	-1.9%
AnchorNav	93.0% 94.4%	91.7%	78.1% 77.9%	+1.4%
CTAForm	91.3% 90.5%	100.0%	100.0%	-0.8%
SubNav	85.2% 86.0%	84.8%	78.9% 80.9%	+2.0%
FormControl	93.0% 93.2%	100.0%	91.5% 83.1%	-8.4%
SubdomainNavBar	71.8% 71.9%	79.4%	70.3% 70.1%	-0.2%
Statistic	97.3%	100.0%	91.4% 89.2%	-2.2%
Tooltip	67.5%	52.9%	58.7% 58.3%	-0.4%

[danielguillan]

LGTM!