feat(ui,react): Add shared React variant to reduce bundle size

[brkalow]

Summary

• Introduces a new ui.shared.browser.js build variant that externalizes React dependencies, allowing the host application's React to be reused instead of bundling a separate copy
• Adds @clerk/ui/register module to register React on globalThis.__clerkSharedModules for sharing with @clerk/ui
• Adds clerkUiVariant option to explicitly control which variant to use
• For @clerk/react users, the shared variant is automatically detected and enabled for compatible React versions
• Extracts version checking logic into a testable utility with comprehensive unit tests
• Adds development warnings for configuration fallbacks and React version mismatches

Test plan

• Unit tests added for version parsing and bounds checking (20 new tests)
• All existing tests pass (328 total)
• Build succeeds for @clerk/react package
• Manual testing: verify bundle size reduction in a sample app using @clerk/react
• Manual testing: verify fallback to non-shared variant when React version is incompatible

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Shared React variant for @clerk/ui that reduces bundle size by reusing the host app’s React.
  • New register module to register host React for use by the UI bundle.
  • @clerk/react will auto-enable the shared variant when the host React is compatible.
  • New clerkUiVariant: 'shared' option for custom integrations (load the register module before the UI bundle).

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[changeset-bot]

🦋 Changeset detected

Latest commit: 6818b53

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

This PR includes changesets to release 20 packages

Name	Type
@clerk/ui	Minor
@clerk/react	Minor
@clerk/shared	Patch
@clerk/astro	Patch
@clerk/chrome-extension	Patch
@clerk/vue	Patch
@clerk/expo	Patch
@clerk/nextjs	Patch
@clerk/react-router	Patch
@clerk/tanstack-react-start	Patch
@clerk/agent-toolkit	Patch
@clerk/backend	Patch
@clerk/clerk-js	Patch
@clerk/expo-passkeys	Patch
@clerk/express	Patch
@clerk/fastify	Patch
@clerk/localizations	Patch
@clerk/msw	Patch
@clerk/nuxt	Patch
@clerk/testing	Patch

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Project	Deployment	Review	Updated (UTC)
clerk-js-sandbox	Ready	Preview, Comment	Jan 16, 2026 5:00am

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a shared React variant for @clerk that externalizes React to reduce duplicate bundling and introduces a ui.shared.browser.js build. Adds @clerk/ui/register (CJS and ESM) which populates globalThis.__clerkSharedModules with React, ReactDOM, and JSX runtime. Introduces version utilities (parseVersion, checkVersionAgainstBounds, isVersionCompatible, IS_REACT_SHARED_VARIANT_COMPATIBLE) and makes @clerk/react auto-enable the shared variant when compatible. Exposes a clerkUiVariant option on IsomorphicClerkOptions and loadClerkJsScript. Build configs (rspack/tsup) and package exports are updated to produce and expose the shared variant.

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