ci: Use fake from main for PR from public fork (#456)

Author: razor-x

.github/workflows/chromatic.yml

@@ -22,11 +22,11 @@ jobs:
       - name: Setup
         uses: ./.github/actions/setup
       - name: Wait for Vercel
-        if: github.run_attempt == '1'
+        if: github.run_attempt == '1' && github.event.pull_request.head.repo.full_name == github.event.pull_request.base.repo.full_name
         run: sleep 20
       - name: Get Vercel preview
         uses: ViktorJT/[email protected]
-        if: github.ref_name != 'main'
+        if: github.ref_name != 'main' && github.event.pull_request.head.repo.full_name == github.event.pull_request.base.repo.full_name
         id: vercel
         with:
           gh_token: ${{ secrets.GITHUB_TOKEN }}
@@ -38,13 +38,15 @@ jobs:
         id: seam_endpoint
         env:
           REF_NAME: ${{ github.ref_name }}
+          HEAD_REPO: ${{ github.event.pull_request.head.repo.full_name }}
+          BASE_REPO: ${{ github.event.pull_request.base.repo.full_name }}
           STORYBOOK_SEAM_ENDPOINT: ${{ vars.STORYBOOK_SEAM_ENDPOINT }}
           VERCEL_PREVIEW_URL: ${{ steps.vercel.outputs.preview_url }}
         run: |
-          if [ $REF_NAME != 'main' ]; then
-            echo "url=${VERCEL_PREVIEW_URL}" >> $GITHUB_OUTPUT
-          else
+          if [ $REF_NAME == 'main' || $HEAD_REPO != $BASE_REPO ]; then
             echo "url=${STORYBOOK_SEAM_ENDPOINT}" >> $GITHUB_OUTPUT
+          else
+            echo "url=${VERCEL_PREVIEW_URL}" >> $GITHUB_OUTPUT
           fi
       - name: Publish to Chromatic
         uses: chromaui/action@v1

README.md

@@ -342,6 +342,16 @@ If changes are detected, follow the link in the status checks to approve the cha
 _Contributors must be granted access to approve changes on Chromatic;
 request access from another maintainer of this project._
 
+##### Note on PR from forks
+
+Pull requests from a public fork do not have access to the
+Vercel secrets required to retrieve the preview deployment URL.
+For these PRs, the Chromatic Storybook and Chromatic UI tests will run against the
+fake version and fake seed from the main branch.
+
+_If your PR is from a public fork and requires updates to the fake version or seed,
+reach out to a project maintainer for assistance._
+
 [Chromatic]: https://www.chromatic.com/
 
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