avoid merging global css in a way that leaks into other chunk groups

packages/next/src/build/webpack/plugins/css-chunking-plugin.ts

@@ -11,6 +11,10 @@ const MIN_CSS_CHUNK_SIZE = 30 * 1024
  */
 const MAX_CSS_CHUNK_SIZE = 100 * 1024
 
+function isGlobalCss(module: Module) {
+  return !/\.module\.(css|scss|sass)$/.test(module.nameForCondition() || '')
+}
+
 type ChunkState = {
   chunk: Chunk
   modules: Module[]
@@ -125,6 +129,8 @@ export class CssChunkingPlugin {
 
           // Process through all modules
           for (const startModule of remainingModules) {
+            let globalCssMode = isGlobalCss(startModule)
+
             // The current position of processing in all selected chunks
             let allChunkStates = new Map(chunkStatesByModule.get(startModule)!)
 
@@ -225,8 +231,36 @@ export class CssChunkingPlugin {
                     }
                   }
                 }
+
+                // Global CSS must not leak into unrelated chunks
+                const nextIsGlobalCss = isGlobalCss(nextModule)
+                if (nextIsGlobalCss && globalCssMode) {
+                  if (allChunkStates.size !== nextChunkStates.size) {
+                    // Fast check
+                    continue
+                  }
+                }
+                if (globalCssMode) {
+                  for (const chunkState of nextChunkStates.keys()) {
+                    if (!allChunkStates.has(chunkState)) {
+                      // Global CSS would leak into chunkState
+                      continue loop
+                    }
+                  }
+                }
+                if (nextIsGlobalCss) {
+                  for (const chunkState of allChunkStates.keys()) {
+                    if (!nextChunkStates.has(chunkState)) {
+                      // Global CSS would leak into chunkState
+                      continue loop
+                    }
+                  }
+                }
                 potentialNextModules.delete(nextModule)
                 currentSize += size
+                if (nextIsGlobalCss) {
+                  globalCssMode = true
+                }
                 for (const [chunkState, i] of nextChunkStates) {
                   if (allChunkStates.has(chunkState)) {
                     // This reduces the request count of the chunk group

test/e2e/app-dir/css-order/app/base.css

@@ -0,0 +1,4 @@
+#hello1,
+#hello2 {
+  color: rgb(255, 0, 0);
+}

test/e2e/app-dir/css-order/app/global-first/page.tsx

@@ -0,0 +1,12 @@
+import '../base.css'
+import './style.css'
+import Nav from '../nav'
+
+export default function Page() {
+  return (
+    <div>
+      <p id="hello1">hello world</p>
+      <Nav />
+    </div>
+  )
+}

test/e2e/app-dir/css-order/app/global-first/style.css

@@ -0,0 +1,4 @@
+#hello1,
+#hello2 {
+  color: rgb(0, 255, 0);
+}

test/e2e/app-dir/css-order/app/global-second/page.tsx

@@ -0,0 +1,12 @@
+import '../base.css'
+import './style.css'
+import Nav from '../nav'
+
+export default function Page() {
+  return (
+    <div>
+      <p id="hello2">hello world</p>
+      <Nav />
+    </div>
+  )
+}

test/e2e/app-dir/css-order/app/global-second/style.css

@@ -0,0 +1,4 @@
+#hello1,
+#hello2 {
+  color: rgb(0, 0, 255);
+}

test/e2e/app-dir/css-order/app/nav.tsx

@@ -70,6 +70,16 @@ export default function Nav() {
             Partial Reversed B
           </Link>
         </li>
+        <li>
+          <Link href={'/global-first'} id="global-first">
+            Global First
+          </Link>
+        </li>
+        <li>
+          <Link href={'/global-second'} id="global-second">
+            Global Second
+          </Link>
+        </li>
       </ul>
       <h3>Pages</h3>
       <ul>

test/e2e/app-dir/css-order/css-order.test.ts

@@ -183,6 +183,20 @@ const PAGES: Record<
     color: 'rgb(255, 55, 255)',
     background: 'rgba(0, 0, 0, 0)',
   },
+  'global-first': {
+    group: 'global',
+    conflict: true,
+    url: '/global-first',
+    selector: '#hello1',
+    color: 'rgb(0, 255, 0)',
+  },
+  'global-second': {
+    group: 'global',
+    conflict: true,
+    url: '/global-second',
+    selector: '#hello2',
+    color: 'rgb(0, 0, 255)',
+  },
 }
 
 const allPairs = getPairs(Object.keys(PAGES))