ci(test): add diff-build job (#29042)

Compares build output between PR and base.

Author: caugner

.github/workflows/test.yml

@@ -5,7 +5,6 @@ on:
     branches:
       - main
   pull_request:
-  merge_group:
 
 permissions:
   contents: read
@@ -18,11 +17,13 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - name: Checkout
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           persist-credentials: false
 
-      - uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
+      - name: Setup Node
+        uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
         with:
           node-version-file: ".nvmrc"
           cache: npm
@@ -40,11 +41,13 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - name: Checkout
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           persist-credentials: false
 
-      - uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
+      - name: Setup Node
+        uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
         with:
           node-version-file: ".nvmrc"
           cache: npm
@@ -58,11 +61,13 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - name: Checkout
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           persist-credentials: false
 
-      - uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
+      - name: Setup Node
+        uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
         with:
           node-version-file: ".nvmrc"
           cache: npm
@@ -71,3 +76,66 @@ jobs:
       - run: npm ci
 
       - run: npm run unittest
+
+  diff-build:
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-latest
+
+    concurrency:
+      group: ${{ github.workflow }}-${{ github.event.pull_request.number }}-diff-build
+      cancel-in-progress: true
+
+    steps:
+      - name: Determine base ref
+        id: base
+        run: |
+          if [[ "$GITHUB_HEAD_REF" == "release" ]]; then
+            VERSION=$(npm view @mdn/browser-compat-data version)
+            echo "ref=v$VERSION" >> "$GITHUB_OUTPUT"
+            echo "Comparing release branch against published v$VERSION"
+          else
+            echo "ref=$GITHUB_BASE_REF" >> "$GITHUB_OUTPUT"
+            echo "Comparing against base branch: $GITHUB_BASE_REF"
+          fi
+
+      # Base
+
+      - name: Checkout (base)
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          ref: ${{ steps.base.outputs.ref }}
+          path: base
+          persist-credentials: false
+
+      - name: Checkout (PR)
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          path: pr
+          persist-credentials: false
+
+      - name: Setup Node
+        uses: actions/setup-node@6044e13b5dc448c55e2357c09f80417699197238 # v6.2.0
+        with:
+          node-version-file: pr/.nvmrc
+          cache: npm
+          cache-dependency-path: |
+            base/package-lock.json
+            pr/package-lock.json
+          package-manager-cache: true
+
+      - name: Install
+        run: |
+          # sed command prefixes each line of output with [base] or [pr]
+          cd base && npm ci | sed "s/^/[base] /" &
+          cd pr && npm ci | sed "s/^/[pr] /" &
+          wait
+
+      - name: Format
+        run: |
+          # sed command prefixes each line of output with [base] or [pr]
+          cd base/build && npx prettier --write . | sed "s/^/[base] /" &
+          cd pr/build && npx prettier --write . | sed "s/^/[pr] /" &
+          wait
+
+      - name: Diff
+        run: git diff --color=always --no-index base/build/ pr/build/ || true