Prevent accidental CSS inclusion in dev (#7381)

* Prevent accidental CSS inclusion in dev

* Use ssrTransformResult.deps instead

* Fix types

* Get dynamic deps too

* Handle virtual module deps

* Fix test on windows

4 files changed, 90 insertions, 1 deletions

diff --git a/.changeset/tiny-lemons-sit.md b/.changeset/tiny-lemons-sit.md
new file mode 100644
index 000000000..8e43429f7
--- /dev/null
+++ b/.changeset/tiny-lemons-sit.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Prevent accidental inclusion of page CSS in dev mode
diff --git a/packages/astro/src/core/module-loader/loader.ts b/packages/astro/src/core/module-loader/loader.ts
index 84bb862d0..39c24253e 100644
--- a/packages/astro/src/core/module-loader/loader.ts
+++ b/packages/astro/src/core/module-loader/loader.ts
@@ -42,6 +42,10 @@ export interface ModuleNode {
 	id: string | null;
 	url: string;
 	ssrModule: Record<string, any> | null;
+	ssrTransformResult: {
+		deps?: string[];
+		dynamicDeps?: string[];
+	} | null;
 	ssrError: Error | null;
 	importedModules: Set<ModuleNode>;
 }
diff --git a/packages/astro/src/core/render/dev/vite.ts b/packages/astro/src/core/render/dev/vite.ts
index fe4d3f791..5cfea6e49 100644
--- a/packages/astro/src/core/render/dev/vite.ts
+++ b/packages/astro/src/core/render/dev/vite.ts
@@ -41,6 +41,7 @@ export async function* crawlGraph(
 			continue;
 		}
 		if (id === entry.id) {
+			const urlDeps = getDepsFromEntry(entry);
 			scanned.add(id);
 			const entryIsStyle = isCSSRequest(id);
 
@@ -82,7 +83,7 @@ export async function* crawlGraph(
 						}
 					}
 				}
-				if (!isPropagationStoppingPoint) {
+				if (urlDeps.includes(urlId(importedModule.url)) && !isPropagationStoppingPoint) {
 					importedModules.add(importedModule);
 				}
 			}
@@ -100,3 +101,19 @@ export async function* crawlGraph(
 		yield* crawlGraph(loader, importedModule.id, false, scanned);
 	}
 }
+
+// Virtual modules URL should start with /@id/ but do not
+function urlId(url: string) {
+	if (url.startsWith('astro:scripts')) {
+		return '/@id/' + url;
+	}
+	return url;
+}
+
+function getDepsFromEntry(entry: ModuleNode) {
+	let deps = entry.ssrTransformResult?.deps ?? [];
+	if(entry.ssrTransformResult?.dynamicDeps) {
+		return deps.concat(entry.ssrTransformResult.dynamicDeps);
+	}
+	return deps;
+}
diff --git a/packages/astro/test/units/dev/styles.test.js b/packages/astro/test/units/dev/styles.test.js
new file mode 100644
index 000000000..44dbc3be5
--- /dev/null
+++ b/packages/astro/test/units/dev/styles.test.js
@@ -0,0 +1,63 @@
+import { expect } from 'chai';
+import { fileURLToPath } from 'url';
+
+import {
+	getStylesForURL
+} from '../../../dist/core/render/dev/css.js';
+import { viteID } from '../../../dist/core/util.js';
+
+const root = new URL('../../fixtures/alias/', import.meta.url);
+
+class TestLoader {
+	constructor(modules) {
+		this.modules = new Map(modules.map(m => [m.id, m]))
+	}
+	getModuleById(id) {
+		return this.modules.get(id);
+	}
+	getModulesByFile(id) {
+		return this.modules.has(id) ? [this.modules.get(id)] : [];
+	}
+}
+
+describe('Crawling graph for CSS', () => {
+	let loader;
+	before(() => {
+		const indexId = viteID(new URL('./src/pages/index.astro', root));
+		const aboutId = viteID(new URL('./src/pages/about.astro', root));
+		loader = new TestLoader([
+			{
+				id: indexId,
+				importedModules: [{
+					id: aboutId,
+					url: aboutId,
+				}, {
+					id: indexId + '?astro&style.css',
+					url: indexId + '?astro&style.css',
+					ssrModule: {}
+				}],
+				ssrTransformResult: {
+					deps: [indexId + '?astro&style.css']
+				}
+			},
+			{
+				id: aboutId,
+				importedModules: [{
+					id: aboutId + '?astro&style.css',
+					url: aboutId + '?astro&style.css',
+					ssrModule: {}
+				}],
+				ssrTransformResult: {
+					deps: [aboutId + '?astro&style.css']
+				}
+			}
+		]);
+	})
+
+	it('importedModules is checked against the child\'s importers', async () => {
+		// In dev mode, HMR modules tracked are added to importedModules. We use `importers`
+		// to verify that they are true importers.
+		const res = await getStylesForURL(new URL('./src/pages/index.astro', root), loader, 'development')
+		expect(res.urls.size).to.equal(1);
+	})
+})