fix: also filter route-owned CSS from root route's entry assets

When Rolldown hoists route chunks into the entry's static imports,
route-owned CSS also leaks into __root__.assets via getChunkCssAssets
recursive traversal, causing every page to load all route CSS.

- Collect route-owned CSS paths into routeChunkFileNames alongside JS
- Filter getChunkCssAssets for root route the same way as getChunkPreloads
- Add test: route-owned CSS does not leak into root assets

Author: ZacharyL2

packages/start-plugin-core/src/start-manifest-plugin/manifestBuilder.ts

@@ -395,25 +395,33 @@ export function buildRouteManifestRoutes(options: {
     }
   }
 
-  // Collect filenames of chunks that belong to specific routes so we can
-  // exclude them from the root route's entry-chunk preloads. Without this,
-  // when Rolldown moves route chunks into the entry's static `imports`
-  // (common in large projects), every route chunk leaks into root preloads
-  // and gets modulepreloaded on every page.
+  // Collect filenames of chunks and CSS that belong to specific routes so we
+  // can exclude them from the root route's entry-chunk preloads and assets.
+  // Without this, when Rolldown moves route chunks into the entry's static
+  // `imports` (common in large projects), every route chunk leaks into root
+  // preloads and every route-owned CSS leaks into root assets.
   const routeChunkFileNames = new Set<string>()
   for (const chunks of options.routeChunksByFilePath.values()) {
     for (const chunk of chunks) {
       routeChunkFileNames.add(
         options.assetResolvers.getAssetPath(chunk.fileName),
       )
+      for (const cssFile of chunk.css) {
+        routeChunkFileNames.add(
+          options.assetResolvers.getAssetPath(cssFile),
+        )
+      }
     }
   }
 
   const rootRoute = (routes[rootRouteId] = routes[rootRouteId] || {})
   mergeRouteChunkData({
     route: rootRoute,
     chunk: options.entryChunk,
-    getChunkCssAssets,
+    getChunkCssAssets: (chunk) => {
+      const assets = getChunkCssAssets(chunk)
+      return assets.filter((a) => !routeChunkFileNames.has(a.attrs.href))
+    },
     getChunkPreloads: (chunk) => {
       const preloads = options.assetResolvers.getChunkPreloads(chunk)
       return preloads.filter((p) => !routeChunkFileNames.has(p))

packages/start-plugin-core/tests/start-manifest-plugin/manifestBuilder.test.ts

@@ -1156,7 +1156,7 @@ describe('multi-chunk routes must merge assets and preloads', () => {
   })
 })
 
-describe('root route should not include route-owned preloads', () => {
+describe('root route should not include route-owned preloads or assets', () => {
   test('entry chunk that statically imports route chunks does not leak them into root preloads', () => {
     const routeChunk = makeChunk({
       fileName: 'about-chunk.js',
@@ -1231,6 +1231,51 @@ describe('root route should not include route-owned preloads', () => {
     // Route chunk should be filtered out from root preloads
     expect(rootPreloads).not.toContain('/assets/about-chunk.js')
   })
+
+  test('route-owned CSS does not leak into root assets', () => {
+    const routeChunk = makeChunk({
+      fileName: 'about-chunk.js',
+      moduleIds: ['/routes/about.tsx?tsr-split=component'],
+      importedCss: ['about.css'],
+    })
+    // Entry chunk statically imports the route chunk (happens in large projects with Rolldown)
+    const entryChunk = makeChunk({
+      fileName: 'entry.js',
+      isEntry: true,
+      imports: ['about-chunk.js'],
+      importedCss: ['entry.css'],
+    })
+
+    const manifest = buildStartManifest({
+      clientBuild: normalizeViteClientBuild({
+        'entry.js': entryChunk,
+        'about-chunk.js': routeChunk,
+      }),
+      routeTreeRoutes: {
+        __root__: { children: ['/about'] } as any,
+        '/about': { filePath: '/routes/about.tsx' },
+      },
+      basePath: '/assets',
+    })
+
+    const rootAssets = manifest.routes['__root__']?.assets ?? []
+    const aboutAssets = manifest.routes['/about']?.assets ?? []
+
+    // Route CSS should be in its own route's assets
+    expect(aboutAssets).toContainEqual(
+      expect.objectContaining({ attrs: expect.objectContaining({ href: '/assets/about.css' }) }),
+    )
+
+    // Route CSS should NOT leak into root assets
+    expect(rootAssets).not.toContainEqual(
+      expect.objectContaining({ attrs: expect.objectContaining({ href: '/assets/about.css' }) }),
+    )
+
+    // Entry CSS should still be in root assets
+    expect(rootAssets).toContainEqual(
+      expect.objectContaining({ attrs: expect.objectContaining({ href: '/assets/entry.css' }) }),
+    )
+  })
 })
 
 describe('buildStartManifest route pruning', () => {