Fix server CSS imports and HMR not working properly in specific condi…

…tions (#49462)

In Hot Reloader we use `mod.resource.replace(/\\/g, '/').includes(key)`
to see if the module is the entry. However that `includes` check isn't
solid. Say the entry key is `app/path/page`, these will all be matched:

```
app/path/page.js
app/path/page.css
app/unrelated/app/path/page/another/page.js
```

This PR fixes that and added a test case. I'm unsure if this fixes the
newly reported cases in #43396.

fix NEXT-1110

packages/next/src/server/dev/hot-reloader.ts

@@ -885,6 +885,10 @@ export default class HotReloader {
     const prevEdgeServerPageHashes = new Map<string, string>()
     const prevCSSImportModuleHashes = new Map<string, string>()
 
+    const pageExtensionRegex = new RegExp(
+      `\\.(?:${this.config.pageExtensions.join('|')})$`
+    )
+
     const trackPageChanges =
       (
         pageHashMap: Map<string, string>,
@@ -912,7 +916,9 @@ export default class HotReloader {
                   modsIterable.forEach((mod: any) => {
                     if (
                       mod.resource &&
-                      mod.resource.replace(/\\/g, '/').includes(key)
+                      mod.resource.replace(/\\/g, '/').includes(key) &&
+                      // Shouldn't match CSS modules, etc.
+                      pageExtensionRegex.test(mod.resource)
                     ) {
                       // use original source to calculate hash since mod.hash
                       // includes the source map in development which changes

test/e2e/app-dir/app-css/app/hmr/import/actual-styles.css

@@ -0,0 +1,3 @@
+body {
+  background-color: red;
+}

test/e2e/app-dir/app-css/app/hmr/import/page.css

@@ -0,0 +1 @@
+@import './actual-styles.css';

test/e2e/app-dir/app-css/app/hmr/import/page.js

@@ -0,0 +1,5 @@
+import './page.css'
+
+export default function Page() {
+  return <div>hello!</div>
+}

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

@@ -301,6 +301,40 @@ createNextDescribe(
           }
         })
 
+        it('should reload @import styles during HMR', async () => {
+          const filePath = 'app/hmr/import/actual-styles.css'
+          const origContent = await next.readFile(filePath)
+
+          // background should be red
+          const browser = await next.browser('/hmr/import')
+          expect(
+            await browser.eval(
+              `window.getComputedStyle(document.querySelector('body')).backgroundColor`
+            )
+          ).toBe('rgb(255, 0, 0)')
+
+          try {
+            await next.patchFile(
+              filePath,
+              origContent.replace(
+                'background-color: red;',
+                'background-color: blue;'
+              )
+            )
+
+            // Wait for HMR to trigger
+            await check(
+              () =>
+                browser.eval(
+                  `window.getComputedStyle(document.querySelector('body')).backgroundColor`
+                ),
+              'rgb(0, 0, 255)'
+            )
+          } finally {
+            await next.patchFile(filePath, origContent)
+          }
+        })
+
         describe('multiple entries', () => {
           it('should only inject the same style once if used by different layers', async () => {
             const browser = await next.browser('/css/css-duplicate-2/client')