chore: merged Jest changedSince testing into main PR Jest job

.github/workflows/frontend.yml

@@ -29,8 +29,11 @@ jobs:
       typecheckable_rules_changed: ${{ steps.changes.outputs.typecheckable_rules_changed }}
       mdx_typecheckable: ${{ steps.changes.outputs.mdx_typecheckable }}
       frontend_all: ${{ steps.changes.outputs.frontend_all }}
+      merge_base: ${{ steps.merge_base.outputs.merge_base }}
     steps:
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+        with:
+          fetch-depth: 100
 
       - name: Check for frontend file changes
         uses: dorny/paths-filter@0bc4621a3135347011ad047f9ecf449bf72ce2bd # v3.0.0
@@ -40,6 +43,28 @@ jobs:
           filters: .github/file-filters.yml
           list-files: shell
 
+      # On PRs, HEAD is the merge commit; its parents (HEAD^1, HEAD^2) are base and head.
+      # Merge base of those two is what Jest --changedSince needs.
+      # If merge base can't be computed or non-frontend files changed, output is empty
+      # and the Jest job will run the full test suite instead.
+      - name: Get merge base for changedSince
+        id: merge_base
+        if: github.event_name == 'pull_request'
+        run: |
+          MERGE_BASE=$(git merge-base HEAD^1 HEAD^2 2>/dev/null) || true
+          if [ -n "$MERGE_BASE" ]; then
+            CHANGED=$(git diff --name-only "$MERGE_BASE" HEAD^2)
+            if echo "$CHANGED" | grep -qvE '^static/'; then
+              echo "Non-frontend file changed — running full Jest suite"
+              MERGE_BASE=""
+            else
+              echo "Merge base: $MERGE_BASE (Jest will use --changedSince)"
+            fi
+          else
+            echo "Could not compute merge base — running full Jest suite"
+          fi
+          echo "merge_base=${MERGE_BASE:-}" >> "$GITHUB_OUTPUT"
+
   typescript:
     if: needs.files-changed.outputs.frontend_all == 'true'
     needs: files-changed
@@ -144,6 +169,9 @@ jobs:
     steps:
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
         name: Checkout sentry
+        with:
+          # PRs need history so Jest can resolve --changedSince <merge_base> from local Git history.
+          fetch-depth: ${{ github.event_name == 'pull_request' && '100' || '1' }}
 
       - uses: ./.github/actions/setup-node-pnpm
 
@@ -189,6 +217,9 @@ jobs:
           # When the "Frontend: Rerun Flaky Tests" label is on the PR,
           # tests wrapped with it.isKnownFlake() run 50x to validate fixes.
           RERUN_KNOWN_FLAKY_TESTS: "${{ contains(github.event.pull_request.labels.*.name, 'Frontend: Rerun Flaky Tests') }}"
+          # When set, Jest uses --changedSince to run only tests affected by this PR.
+          # Empty on master or when non-frontend files changed (falls back to full suite).
+          MERGE_BASE: ${{ needs.files-changed.outputs.merge_base }}
         run: pnpm run test-ci --forceExit
 
   form-field-registry: