Add ESM bundle support

[rezrah]

Summary

Towards https://github.com/github/brand-experience/issues/80

Adds an ESM entrypoint for @primer/react-brand so that modern UI frameworks and bundlers can import Primer Brand components in a more efficient way.

The previous UMD bundle and import mechanisms are still in-place and default. No breaking changes are proposed as part of this change.

The ESM bundles will allow users of the package to import only the components and styles they need, rather than the entire library. It also simplifies the way styles are loaded, which is now performed automatically.

Eventually ESM will become the default, and we will remove the UMD + Webpack legacy bundles. This will happen once we ship the v1 release of Primer Brand.

Build times (cold start)
Webpack (UMD)	Vite (ESM)
10.1s	1.1s

CSS import size for Button component
Before: UMD	After: ESM
720 kB CSS	52 kB CSS

List of notable changes:

• Added Vite 8 to create a new entrypoint at @primer/react-brand/esm, which users can opt-into.
  • Note: Bundlers won't pick this up automatically to avoid a breaking change. This will be deferred to the v1 release.
• Updated the Getting Started guide to include new documentation on ESM bundles
• Fixed integration tests for Astro
• Updated integration tests for Next.js to run in matrix strategy and test both UMD and the new ESM bundles
• Added a new bundle size comment to PRs for future use

What should reviewers focus on?

• Check the CI checks pass first
• Take the canary package and install it into a new Next.js or Tanstack Start installation using ESM guide
• Verify that the instructions are valid

Steps to test:

1. Go to this updated documentation guide
2. Install the canary package from this repo in your own, separate ESM-compatible framework
3. Verify it works as expected

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

• Vite 8 docs

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:

New bundle size comment in PRs (scroll down to see the real version):

[changeset-bot]

🦋 Changeset detected

Latest commit: f07b68b

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

This PR includes changesets to release 8 packages

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

[github-actions]

🟢 Bundle size report

Check	Main	Branch	Change
UMD — full bundle (JS)	0 B	96.58 kB	🆕 96.58 kB
UMD — full bundle (CSS)	0 B	63.34 kB	🆕 63.34 kB
ESM — full bundle (JS + CSS)	0 B	1.45 MB	🆕 1.45 MB
ESM — tree-shaken simple (Button)	0 B	66.45 kB	🆕 66.45 kB
ESM — tree-shaken complex (ActionMenu)	0 B	75.26 kB	🆕 75.26 kB

[github-actions]

🟢 No visual differences found

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

[github-actions]

🟢 No unit test coverage changes found

All components and hooks with tests maintain the same coverage as the main branch.

[Copilot]

Pull request overview

Adds an ESM build/output pathway for @primer/react-brand alongside the existing UMD build, plus CI/docs updates to support and validate the new distribution and bundle-size reporting.

Changes:

• Added a Vite-based ESM build (packages/react/esm) with a new @primer/react-brand/esm export path and corresponding TypeScript declarations.
• Introduced bundle size checks (Size Limit + GitHub Action) and a script to generate a PR comment report.
• Updated Next.js integration tests and docs to exercise/describe UMD vs ESM consumption paths.

Reviewed changes

Copilot reviewed 23 out of 26 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
packages/repo-configs/scripts/bundle-size-report.js	Generates PR comment output for Size Limit comparisons.
packages/react/vite.esm.config.ts	Adds Vite build config for the ESM output, including CSS module naming and CSS injection.
packages/react/tsconfig.esm.json	Emits ESM declaration files into esm/.
packages/react/src/index.esm.ts	Adds an ESM entry that re-exports the main index.
packages/react/src/css/stylesheets.ts	Adds a noop export intended to prevent global styles from being tree-shaken away.
packages/react/src/LogoSuite/LogoSuite.tsx	Switches play/pause icons to Octicons components.
packages/react/scripts/postbuild	Copies LICENSE into esm/ output.
packages/react/package.json	Adds exports map entries for /esm, adds ESM build script, and updates sideEffects/files.
packages/react/.gitignore	Ignores esm/ output.
packages/e2e/integration-tests/fixtures/index.umd.ts	Updates UMD fixture imports and ensures global CSS is loaded for UMD case.
packages/e2e/integration-tests/fixtures/index.esm.ts	Switches fixture imports to @primer/react-brand/esm.
packages/e2e/integration-tests/fixtures/KitchenSink.tsx	Removes global CSS import (now handled in the UMD fixture barrel).
package.json	Adds size script and Size Limit deps.
package-lock.json	Locks new dependencies (Size Limit, Vite-related, Octicons bump, etc.).
apps/next-docs/package.json	Bumps @primer/octicons-react.
apps/next-docs/next-env.d.ts	Changes Next route types reference (currently problematic).
apps/next-docs/content/**	Adds/adjusts docs navigation + new ESM getting-started guide.
.size-limit.js	Defines bundle-size checks for UMD and ESM (including tree-shaken cases).
.gitignore	Ignores esm/ output and adds an integration-test artifact ignore entry (currently mismatched).
.github/workflows/integration_test_nextjs.yml	Runs Next.js integration tests for both UMD and ESM setups via a matrix.
.github/workflows/bundle_size.yml	Adds PR-time bundle-size reporting workflow.
.changeset/free-planets-fold.md	Changeset describing the new ESM entry point and usage.

---

You can also share your feedback on Copilot code review. Take the survey.

[danielguillan]

Great work! Just one minor fix in the changeset and a question regarding sideEffects.