withastro · ematipico · Mar 31, 2026 · Mar 27, 2026 · Mar 27, 2026 · Mar 27, 2026
diff --git a/.changeset/lucky-kiwis-swim.md b/.changeset/lucky-kiwis-swim.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fixes a bug where page-level CSS could leak between unrelated pages when traversing style parents across top-level route boundaries
diff --git a/packages/astro/src/core/build/plugins/plugin-css.ts b/packages/astro/src/core/build/plugins/plugin-css.ts
@@ -30,6 +30,12 @@ interface PluginOptions {
 	buildOptions: StaticBuildOptions;
 }
 
+function isBuildCssBoundary(id: string, ctx: { getModuleInfo: GetModuleInfo }): boolean {
+	if (isPropagatedAssetBoundary(id)) return true;
+	const info = ctx.getModuleInfo(id);
+	return info ? moduleIsTopLevelPage(info) : false;
+}
+
 function rollupPluginAstroBuildCSS(options: PluginOptions): VitePlugin[] {
 	const { internals, buildOptions } = options;
 	const { settings } = buildOptions;
@@ -158,7 +164,7 @@ function rollupPluginAstroBuildCSS(options: PluginOptions): VitePlugin[] {
 								const parentModuleInfos = getParentExtendedModuleInfos(
 									scopedToModule,
 									this,
-									isPropagatedAssetBoundary,
+									(moduleId) => isBuildCssBoundary(moduleId, this),
 								);
 								for (const { info: pageInfo, depth, order } of parentModuleInfos) {
 									if (moduleIsTopLevelPage(pageInfo)) {
@@ -230,7 +236,7 @@ function rollupPluginAstroBuildCSS(options: PluginOptions): VitePlugin[] {
 					const parentModuleInfos = getParentExtendedModuleInfos(
 						id,
 						this,
-						isPropagatedAssetBoundary,
+						(importer) => isBuildCssBoundary(importer, this),
 					);
 					for (const { info: pageInfo, depth, order } of parentModuleInfos) {
 						if (isPropagatedAssetBoundary(pageInfo.id)) {

diff --git a/packages/astro/test/fixtures/i18n-css-leak-basic/astro.config.mjs b/packages/astro/test/fixtures/i18n-css-leak-basic/astro.config.mjs
@@ -0,0 +1,14 @@
+import { defineConfig } from 'astro/config';
+
+export default defineConfig({
+  build: {
+    inlineStylesheets: 'never',
+  },
+  i18n: {
+    locales: ['en'],
+    defaultLocale: 'en',
+    routing: {
+      redirectToDefaultLocale: false,
+    },
+  },
+});
diff --git a/packages/astro/test/fixtures/i18n-css-leak-basic/package.json b/packages/astro/test/fixtures/i18n-css-leak-basic/package.json
@@ -0,0 +1,8 @@
+{
+  "name": "@test/i18n-css-leak-basic",
+  "version": "0.0.0",
+  "private": true,
+  "dependencies": {
+    "astro": "workspace:*"
+  }
+}
diff --git a/packages/astro/test/fixtures/i18n-css-leak-basic/src/components/Header.astro b/packages/astro/test/fixtures/i18n-css-leak-basic/src/components/Header.astro
@@ -0,0 +1,9 @@
+---
+import { getRelativeLocaleUrl } from 'astro:i18n';
+
+const docsHref = getRelativeLocaleUrl('en', 'docs');
+---
+
+<header>
+  <a href={docsHref}>Docs</a>
+</header>
diff --git a/packages/astro/test/fixtures/i18n-css-leak-basic/src/layouts/DocsLayout.astro b/packages/astro/test/fixtures/i18n-css-leak-basic/src/layouts/DocsLayout.astro
@@ -0,0 +1,12 @@
+---
+import '../styles/docs.css';
+---
+
+<html>
+  <head>
+    <title>Docs</title>
+  </head>
+  <body>
+    <slot />
+  </body>
+</html>
diff --git a/packages/astro/test/fixtures/i18n-css-leak-basic/src/layouts/SiteLayout.astro b/packages/astro/test/fixtures/i18n-css-leak-basic/src/layouts/SiteLayout.astro
@@ -0,0 +1,14 @@
+---
+import Header from '../components/Header.astro';
+import '../styles/site.css';
+---
+
+<html>
+  <head>
+    <title>Site</title>
+  </head>
+  <body>
+    <Header />
+    <slot />
+  </body>
+</html>
diff --git a/packages/astro/test/fixtures/i18n-css-leak-basic/src/pages/docs/index.astro b/packages/astro/test/fixtures/i18n-css-leak-basic/src/pages/docs/index.astro
@@ -0,0 +1,7 @@
+---
+import DocsLayout from '../../layouts/DocsLayout.astro';
+---
+
+<DocsLayout>
+  <h1 id="docs-heading">Docs</h1>
+</DocsLayout>
diff --git a/packages/astro/test/fixtures/i18n-css-leak-basic/src/pages/index.astro b/packages/astro/test/fixtures/i18n-css-leak-basic/src/pages/index.astro
@@ -0,0 +1,7 @@
+---
+import SiteLayout from '../layouts/SiteLayout.astro';
+---
+
+<SiteLayout>
+  <h1 id="site-heading">Home</h1>
+</SiteLayout>
diff --git a/packages/astro/test/fixtures/i18n-css-leak-basic/src/styles/docs.css b/packages/astro/test/fixtures/i18n-css-leak-basic/src/styles/docs.css
@@ -0,0 +1,7 @@
+body {
+  background: black;
+}
+
+h1 {
+  color: red;
+}
diff --git a/packages/astro/test/fixtures/i18n-css-leak-basic/src/styles/site.css b/packages/astro/test/fixtures/i18n-css-leak-basic/src/styles/site.css
@@ -0,0 +1,3 @@
+body {
+  background: white;
+}
diff --git a/packages/astro/test/i18n-css-leak.test.js b/packages/astro/test/i18n-css-leak.test.js
@@ -0,0 +1,40 @@
+import assert from 'node:assert/strict';
+import { before, describe, it } from 'node:test';
+import { load as cheerioLoad } from 'cheerio';
+import { loadFixture } from './test-utils.js';
+
+describe('CSS graph boundaries with astro:i18n', () => {
+	let fixture;
+
+	before(async () => {
+		fixture = await loadFixture({
+			root: './fixtures/i18n-css-leak-basic/',
+			build: { inlineStylesheets: 'never' },
+		});
+		await fixture.build();
+	});
+
+	async function getPageCss(pathname) {
+		const html = await fixture.readFile(pathname);
+		const $ = cheerioLoad(html);
+		const hrefs = $('link[rel=stylesheet]')
+			.map((_index, el) => $(el).attr('href'))
+			.get();
+		const stylesheets = await Promise.all(hrefs.map((href) => fixture.readFile(href)));
+		return stylesheets.join('\n');
+	}
+
+	it('does not attach docs-only CSS to unrelated pages', async () => {
+		const css = await getPageCss('/index.html');
+		assert.match(css, /background:#fff/);
+		assert.doesNotMatch(css, /background:#000/);
+		assert.doesNotMatch(css, /color:red/);
+	});
+
+	it('keeps docs-only CSS on the docs page', async () => {
+		const css = await getPageCss('/docs/index.html');
+		assert.match(css, /background:#000/);
+		assert.match(css, /color:red/);
+		assert.doesNotMatch(css, /background:#fff/);
+	});
+});
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml