From cae2d2a870aa59c020fd75a59ec2eefcf24c21f5 Mon Sep 17 00:00:00 2001
From: Alan Agius <17563226+alan-agius4@users.noreply.github.com>
Date: Thu, 1 Oct 2026 07:26:01 +0000
Subject: [PATCH] perf(@angular/build): avoid invalidating Angular diagnostic
 cache on stylesheet change

Component stylesheets (.css, .scss, .sass, .less) do not impact TypeScript semantic diagnostics or Angular template diagnostics.

When a stylesheet dependency changes during watch rebuilds, avoid deleting the component's cached Angular diagnostics or marking the component source file as affected for diagnostics re-collection.
---
 .../build/src/tools/angular/compilation/aot-compilation.ts   | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/packages/angular/build/src/tools/angular/compilation/aot-compilation.ts b/packages/angular/build/src/tools/angular/compilation/aot-compilation.ts
index ca74e721ba7d..e1205db0780a 100644
--- a/packages/angular/build/src/tools/angular/compilation/aot-compilation.ts
+++ b/packages/angular/build/src/tools/angular/compilation/aot-compilation.ts
@@ -216,7 +216,10 @@ export class AotCompilation extends TypeScriptCompilation {
         // Also invalidate Angular diagnostics for a source file if component resources are modified
         if (this.#state && hostOptions.modifiedFiles?.size) {
           for (const resourceDependency of resourceDependencies) {
-            if (hostOptions.modifiedFiles.has(resourceDependency)) {
+            if (
+              hostOptions.modifiedFiles.has(resourceDependency) &&
+              !/\.(?:css|scss|sass|less)$/i.test(resourceDependency)
+            ) {
               this.#state.diagnosticCache.delete(sourceFile);
               // Also mark as affected in case changed template affects diagnostics
               affectedFiles.add(sourceFile);