[devtool] fix overlay styles are missing

packages/next/src/build/webpack/loaders/devtool/devtool-style-inject.js

@@ -85,42 +85,50 @@ function startObservingForPortal() {
 
   // Set up MutationObserver to watch for the portal element
   const observer = new MutationObserver((mutations) => {
-    if (mutations.length === 0 || mutations[0].addedNodes.length === 0) {
+    if (mutations.length === 0) {
       return
     }
 
-    // Check if mutation is script[data-nextjs-dev-overlay] tag, which is the
-    // parent of the nextjs-portal element
-    const mutationNode = mutations[0].addedNodes[0]
-    let portalNode = null
-    if (
-      // app router: body > script[data-nextjs-dev-overlay] > nextjs-portal
-      mutationNode.tagName === 'SCRIPT' &&
-      mutationNode.getAttribute('data-nextjs-dev-overlay')
-    ) {
-      portalNode = mutationNode.firstChild
-    } else if (
-      // pages router: body > nextjs-portal
-      mutationNode.tagName === 'NEXTJS-PORTAL'
-    ) {
-      portalNode = mutationNode
-    }
-    if (!portalNode) {
-      return
-    }
-
-    // Wait until shadow root is available
-    const checkShadowRoot = () => {
-      if (getShadowRoot()) {
-        flushCachedElements()
-        observer.disconnect()
-        cache.isObserving = false
-      } else {
-        // Try again after a short delay
-        setTimeout(checkShadowRoot, 20)
+    // Check all mutations and all added nodes
+    for (const mutation of mutations) {
+      if (mutation.addedNodes.length === 0) continue
+
+      for (const addedNode of mutation.addedNodes) {
+        if (addedNode.nodeType !== Node.ELEMENT_NODE) continue
+
+        const mutationNode = addedNode
+
+        let portalNode = null
+        if (
+          // app router: body > script[data-nextjs-dev-overlay] > nextjs-portal
+          mutationNode.tagName === 'SCRIPT' &&
+          mutationNode.getAttribute('data-nextjs-dev-overlay')
+        ) {
+          portalNode = mutationNode.firstChild
+        } else if (
+          // pages router: body > nextjs-portal
+          mutationNode.tagName === 'NEXTJS-PORTAL'
+        ) {
+          portalNode = mutationNode
+        }
+
+        if (portalNode) {
+          // Wait until shadow root is available
+          const checkShadowRoot = () => {
+            if (getShadowRoot()) {
+              flushCachedElements()
+              observer.disconnect()
+              cache.isObserving = false
+            } else {
+              // Try again after a short delay
+              setTimeout(checkShadowRoot, 20)
+            }
+          }
+          checkShadowRoot()
+          return // Exit early once we find a portal
+        }
       }
     }
-    checkShadowRoot()
   })
 
   observer.observe(document.body, {

test/development/error-overlay/index.test.tsx

@@ -95,4 +95,28 @@ describe('DevErrorOverlay', () => {
       expect(request.status).toBe(200)
     }
   })
+
+  it('should load dev overlay styles successfully', async () => {
+    const browser = await next.browser('/hydration-error')
+
+    await assertHasRedbox(browser)
+    const redbox = browser.locateRedbox()
+
+    // check the data-nextjs-dialog-header="true" DOM element styles under redbox is applied
+    const dialogHeader = redbox.locator('[data-nextjs-dialog-header="true"]')
+    expect(await dialogHeader.isVisible()).toBe(true)
+    // get computed styles
+    const computedStyles = await dialogHeader.evaluate((element) => {
+      return window.getComputedStyle(element)
+    })
+    const styles = {
+      backgroundColor: computedStyles.backgroundColor,
+      color: computedStyles.color,
+    }
+
+    expect(styles).toEqual({
+      backgroundColor: 'rgba(0, 0, 0, 0)',
+      color: 'rgb(117, 117, 117)',
+    })
+  })
 })

test/development/error-overlay/pages/_app.tsx

@@ -0,0 +1,5 @@
+import type { AppProps } from 'next/app'
+
+export default function App({ Component, pageProps }: AppProps) {
+  return <Component {...pageProps} />
+}

test/development/error-overlay/pages/_document.tsx

@@ -0,0 +1,13 @@
+import { Html, Head, Main, NextScript } from 'next/document'
+
+export default function Document() {
+  return (
+    <Html lang="en">
+      <Head />
+      <body>
+        <Main />
+        <NextScript />
+      </body>
+    </Html>
+  )
+}

test/development/error-overlay/pages/hydration-error.tsx

@@ -0,0 +1,5 @@
+export default function Home() {
+  return (
+    <div>{typeof window === 'undefined' ? <p>Server</p> : <p>Client</p>}</div>
+  )
+}