Report the shot at the commit that has a branch

.github/workflows/ci.yml

@@ -191,10 +191,34 @@ jobs:
       # times, and the nightly is what keeps asking. Chromatic answers "should
       # this have changed?", which only makes sense once the answer to "did
       # anything change by itself?" is no.
+      #
+      # `CHROMATIC_SHA`, and the branch and slug that have to travel with it,
+      # because on a `pull_request` event `GITHUB_SHA` is the *merge* commit
+      # GitHub builds for the run -- a commit that exists on no branch. Left
+      # alone, Chromatic posts `UI Tests` and `UI Review` there, where the pull
+      # request shows them (GitHub knows the merge commit is its) but a
+      # required status check cannot: those are read off the head commit,
+      # which is where every Actions job already reports. Measured on #187:
+      # twelve check runs on `5520f5f2`, three Chromatic statuses on the merge
+      # commit `aefcbe4`, no overlap.
+      #
+      # It also picks the wrong baseline. That merge commit has the latest
+      # build on main for an ancestor -- the pull request's own branch does
+      # not -- so every comparison is made against main rather than against
+      # what this branch last looked like.
+      #
+      # Chromatic's own answer is to run this on `push` instead, which for
+      # this workflow would mean the whole 20-minute sweep on every branch
+      # push, pull request or not. So: the documented alternative, verbatim,
+      # falling back to the push values on main.
+      #
       - run: pnpm chromatic ${{ github.ref == 'refs/heads/main' && '--auto-accept-changes' || '--exit-zero-on-changes' }}
         if: ${{ env.CHROMATIC == 'true' }}
         env:
           CHROMATIC_PROJECT_TOKEN: ${{ secrets.CHROMATIC_PROJECT_TOKEN }}
+          CHROMATIC_SHA: ${{ github.event.pull_request.head.sha || github.sha }}
+          CHROMATIC_BRANCH: ${{ github.event.pull_request.head.ref || github.ref_name }}
+          CHROMATIC_SLUG: ${{ github.repository }}
 
   #
   # Build