fix: restore size.yml workflow and update size-limit config for ESM-only build

[Copilot]

PR #6425 removed browser bundles and deleted size.yml, breaking PR bundle size comments. The size-limit config in package.json was also left pointing to the now-nonexistent dist/browser.esm.js and dist/browser.umd.js.

Changelog

New

• Restored .github/workflows/size.yml — triggers on pull_request, runs andresz1/size-limit-action@v1 with build_script: build to handle the monorepo ESM-only build before measuring. Adds explicit permissions: pull-requests: write required for posting PR comments.

Changed

• Updated size-limit config in root package.json to target packages/react/dist/index.js instead of the deleted browser bundles:

"size-limit": [
  {
    "path": "packages/react/dist/index.js",
    "webpack": false,
    "running": false
  }
]

Removed

• Dropped stale size-limit entries for dist/browser.esm.js and dist/browser.umd.js

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

CI/workflow-only change — no package code or public API affected.

Testing & Reviewing

Verify that after merging, new PRs receive a bundle size comparison comment from andresz1/size-limit-action. The sizes job in ci.yml (Job Summary) is unchanged.

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)

Original prompt

Problem

PR #6425 ("feat: only publish esm bundles from @primer/react") deleted the .github/workflows/size.yml workflow and removed the browser bundle outputs (dist/browser.esm.js and dist/browser.umd.js) from the rollup config. This means:

1. No more PR comments with bundle size differences — The size.yml workflow used andresz1/size-limit-action to post size comparison comments on pull requests (e.g., Blankslate: Add button support to PrimaryAction #5687 (comment)). This was deleted in PR feat: only publish esm bundles from @primer/react #6425.

2. Stale size-limit config — The root package.json still has a size-limit config pointing to the now-nonexistent packages/react/dist/browser.esm.js and packages/react/dist/browser.umd.js files.

The sizes job in ci.yml still runs get-export-sizes.cjs and writes to the GitHub Actions Job Summary, but it does NOT post PR comments. The PR comment behavior was exclusively handled by size.yml.

What to Fix

1. Restore .github/workflows/size.yml

Re-create the workflow file at .github/workflows/size.yml. It should:

• Trigger on pull_request events
• Use the andresz1/size-limit-action GitHub Action (use a recent version/SHA)
• Pass github_token: ${{ secrets.GITHUB_TOKEN }}
• Since the package is now ESM-only and in a monorepo, the action needs to build the project first. Add a build_script option with value build so the action runs npm run build before measuring sizes.

Example structure:

name: Size
on: [pull_request]
jobs:
  size:
    name: Size
    runs-on: ubuntu-latest
    env:
      CI_JOB_NUMBER: 1
    steps:
      - uses: actions/checkout@v4
      - uses: andresz1/size-limit-action@v1
        with:
          github_token: ${{ secrets.GITHUB_TOKEN }}
          build_script: build

2. Update size-limit config in root package.json

Update the size-limit array to point to the actual ESM entrypoints that exist after the build. The old browser bundles no longer exist. The new main entrypoint is packages/react/dist/index.js. Update the config to:

"size-limit": [
  {
    "path": "packages/react/dist/index.js",
    "webpack": false,
    "running": false
  }
]

Remove the entries for browser.esm.js and browser.umd.js since those files are no longer produced by the build.

3. Optionally clean up dead dependencies

If there's nothing else using them, the size-limit and @size-limit/preset-big-lib dependencies and the "size": "size-limit" script in root package.json should be kept since they're needed by the size-limit-action.

Context

• The get-export-sizes.cjs script at packages/react/script/get-export-sizes.cjs and the sizes job in ci.yml are separate — they write to Job Summary, not PR comments. Leave them as-is.
• The size-limit-action is what provides the PR comment behavior.
• The rollup config now only produces ESM output to the dist/ directory (no more lib/, lib-esm/, or dist/browser.*.js).
• Make sure to run npm run format before committing to pass CI formatting checks.

This pull request was created from Copilot chat.

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[github-actions]

size-limit report 📦

Path	Size
	0 B (+100% 🔺)
packages/react/dist/index.js	990 B (+100% 🔺)

[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. Or, apply the integration-tests: skipped manually label to skip these checks.