fix(@angular/build): workaround Vite CSS ShadowDOM hot replacement

When using the development server with the application builder (default for new projects),
Angular components using ShadowDOM view encapsulation will now cause a full page reload.
This ensures that these components styles are correctly updated during watch mode. Vite's
CSS hot replacement client code currently does not support searching and replacing `<link>`
elements inside shadow roots. When support is available within Vite, an HMR based update
for ShadowDOM components can be supported as other view encapsulation modes are now.

packages/angular/build/src/builders/dev-server/vite-server.ts

@@ -406,6 +406,7 @@ export async function* serveWithVite(
             key: 'r',
             description: 'force reload browser',
             action(server) {
+              usedComponentStyles.clear();
               server.ws.send({
                 type: 'full-reload',
                 path: '*',
@@ -433,7 +434,7 @@ async function handleUpdate(
   server: ViteDevServer,
   serverOptions: NormalizedDevServerOptions,
   logger: BuilderContext['logger'],
-  usedComponentStyles: Map<string, Set<string>>,
+  usedComponentStyles: Map<string, Set<string | boolean>>,
 ): Promise<void> {
   const updatedFiles: string[] = [];
   let destroyAngularServerAppCalled = false;
@@ -469,42 +470,57 @@ async function handleUpdate(
 
   if (serverOptions.hmr) {
     if (updatedFiles.every((f) => f.endsWith('.css'))) {
+      let requiresReload = false;
       const timestamp = Date.now();
-      server.ws.send({
-        type: 'update',
-        updates: updatedFiles.flatMap((filePath) => {
-          // For component styles, an HMR update must be sent for each one with the corresponding
-          // component identifier search parameter (`ngcomp`). The Vite client code will not keep
-          // the existing search parameters when it performs an update and each one must be
-          // specified explicitly. Typically, there is only one each though as specific style files
-          // are not typically reused across components.
-          const componentIds = usedComponentStyles.get(filePath);
-          if (componentIds) {
-            return Array.from(componentIds).map((id) => ({
-              type: 'css-update',
+      const updates = updatedFiles.flatMap((filePath) => {
+        // For component styles, an HMR update must be sent for each one with the corresponding
+        // component identifier search parameter (`ngcomp`). The Vite client code will not keep
+        // the existing search parameters when it performs an update and each one must be
+        // specified explicitly. Typically, there is only one each though as specific style files
+        // are not typically reused across components.
+        const componentIds = usedComponentStyles.get(filePath);
+        if (componentIds) {
+          return Array.from(componentIds).map((id) => {
+            if (id === true) {
+              // Shadow DOM components currently require a full reload.
+              // Vite's CSS hot replacement does not support shadow root searching.
+              requiresReload = true;
+            }
+
+            return {
+              type: 'css-update' as const,
               timestamp,
-              path: `${filePath}?ngcomp` + (id ? `=${id}` : ''),
+              path: `${filePath}?ngcomp` + (typeof id === 'string' ? `=${id}` : ''),
               acceptedPath: filePath,
-            }));
-          }
+            };
+          });
+        }
 
-          return {
-            type: 'css-update' as const,
-            timestamp,
-            path: filePath,
-            acceptedPath: filePath,
-          };
-        }),
+        return {
+          type: 'css-update' as const,
+          timestamp,
+          path: filePath,
+          acceptedPath: filePath,
+        };
       });
 
-      logger.info('HMR update sent to client(s).');
+      if (!requiresReload) {
+        server.ws.send({
+          type: 'update',
+          updates,
+        });
+        logger.info('HMR update sent to client(s).');
 
-      return;
+        return;
+      }
     }
   }
 
   // Send reload command to clients
   if (serverOptions.liveReload) {
+    // Clear used component tracking on full reload
+    usedComponentStyles.clear();
+
     server.ws.send({
       type: 'full-reload',
       path: '*',

packages/angular/build/src/tools/vite/middlewares/assets-middleware.ts

@@ -15,7 +15,7 @@ export function createAngularAssetsMiddleware(
   server: ViteDevServer,
   assets: Map<string, string>,
   outputFiles: AngularMemoryOutputFiles,
-  usedComponentStyles: Map<string, Set<string>>,
+  usedComponentStyles: Map<string, Set<string | boolean>>,
   encapsulateStyle: (style: Uint8Array, componentId: string) => string,
 ): Connect.NextHandleFunction {
   return function angularAssetsMiddleware(req, res, next) {
@@ -76,14 +76,19 @@ export function createAngularAssetsMiddleware(
         let data: Uint8Array | string = outputFile.contents;
         if (extension === '.css') {
           // Inject component ID for view encapsulation if requested
-          const componentId = new URL(req.url, 'http://localhost').searchParams.get('ngcomp');
+          const searchParams = new URL(req.url, 'http://localhost').searchParams;
+          const componentId = searchParams.get('ngcomp');
           if (componentId !== null) {
-            // Record the component style usage for HMR updates
+            // Track if the component uses ShadowDOM encapsulation (3 = ViewEncapsulation.ShadowDom)
+            const shadow = searchParams.get('e') === '3';
+
+            // Record the component style usage for HMR updates (true = shadow; false = none; string = emulated)
             const usedIds = usedComponentStyles.get(pathname);
+            const trackingId = componentId || shadow;
             if (usedIds === undefined) {
-              usedComponentStyles.set(pathname, new Set([componentId]));
+              usedComponentStyles.set(pathname, new Set([trackingId]));
             } else {
-              usedIds.add(componentId);
+              usedIds.add(trackingId);
             }
 
             // Report if there are no changes to avoid reprocessing