Fix Astro HMR style-only change detection (#10139)

.changeset/twenty-monkeys-thank.md

@@ -0,0 +1,5 @@
+---
+"astro": patch
+---
+
+Fixes style-only change detection for Astro files if both the markup and styles are updated

packages/astro/src/vite-plugin-astro/hmr.ts

@@ -65,7 +65,7 @@ const scriptRE = /<script(?:\s.*?)?>.*?<\/script>/gs;
 // eslint-disable-next-line regexp/no-super-linear-backtracking
 const styleRE = /<style(?:\s.*?)?>.*?<\/style>/gs;
 
-function isStyleOnlyChanged(oldCode: string, newCode: string) {
+export function isStyleOnlyChanged(oldCode: string, newCode: string) {
 	if (oldCode === newCode) return false;
 
 	// Before we can regex-capture style tags, we remove the frontmatter and scripts
@@ -89,9 +89,14 @@ function isStyleOnlyChanged(oldCode: string, newCode: string) {
 	// Finally, we can compare styles
 	const oldStyles: string[] = [];
 	const newStyles: string[] = [];
-	oldCode.match(styleRE)?.forEach((m) => oldStyles.push(m));
-	newCode.match(styleRE)?.forEach((m) => newStyles.push(m));
-	// The length must also be the same for style only change.  If style tags are added/removed,
+	oldCode = oldCode.replace(styleRE, (m) => (oldStyles.push(m), ''));
+	newCode = newCode.replace(styleRE, (m) => (newStyles.push(m), ''));
+
+	// Remaining of `oldCode` and `newCode` is the markup, return false if they're different
+	if (oldCode !== newCode) return false;
+
+	// Finally, check if only the style changed.
+	// The length must also be the same for style only change. If style tags are added/removed,
 	// we need to regenerate the main Astro file so that its CSS imports are also added/removed
 	return oldStyles.length === newStyles.length && !isArrayEqual(oldStyles, newStyles);
 }

packages/astro/test/units/vite-plugin-astro/hmr.test.js

@@ -0,0 +1,63 @@
+import { describe, it } from 'node:test';
+import * as assert from 'node:assert/strict';
+import { isStyleOnlyChanged } from '../../../dist/vite-plugin-astro/hmr.js';
+
+describe('isStyleOnlyChanged', () => {
+	it('should return false if nothing change', () => {
+		const oldCode = 'a';
+		const newCode = 'a';
+		assert.equal(isStyleOnlyChanged(oldCode, newCode), false);
+	});
+
+	it('should return false if script has changed', () => {
+		const oldCode = '<script>console.log("Hello");</script><style>body { color: red; }</style>';
+		const newCode = '<script>console.log("Hi");</script><style>body { color: red; }</style>';
+		assert.equal(isStyleOnlyChanged(oldCode, newCode), false);
+	});
+
+	it('should return true if only style has changed', () => {
+		const oldCode = '<style>body { color: red; }</style>';
+		const newCode = '<style>body { color: blue; }</style>';
+		assert.equal(isStyleOnlyChanged(oldCode, newCode), true);
+	});
+
+	it('should return false if style tags are added or removed', () => {
+		const oldCode = '<style>body { color: red; }</style>';
+		const newCode = '<style>body { color: red; }</style><style>a { color: blue; }</style>';
+		assert.equal(isStyleOnlyChanged(oldCode, newCode), false);
+	});
+
+	it('should return false if frontmatter has changed', () => {
+		const oldCode = `
+---
+title: Hello
+---
+<style>body { color: red; }</style>`;
+		const newCode = `
+---
+title: Hi
+---
+<style>body { color: red; }</style>`;
+		assert.equal(isStyleOnlyChanged(oldCode, newCode), false);
+	});
+
+	it('should return false if both frontmatter and style have changed', () => {
+		const oldCode = `
+---
+title: Hello
+---
+<style>body { color: red; }</style>`;
+		const newCode = `
+---
+title: Hi
+---
+<style>body { color: blue; }</style>`;
+		assert.equal(isStyleOnlyChanged(oldCode, newCode), false);
+	});
+
+	it('should return false if both markup and style have changed', () => {
+		const oldCode = '<h1>Hello</h1><style>body { color: red; }</style>';
+		const newCode = '<h1>Hi</h1><style>body { color: blue; }</style>';
+		assert.equal(isStyleOnlyChanged(oldCode, newCode), false);
+	});
+});