fix: inline server stylesheets instead of client stylesheets (#13068)

fixes #6720

This PR changes the inlined stylesheet from client to server so that the paths for imported assets in the CSS such as fonts are correct when the document first loads. In comparison, the client stylesheet links are relative by default, so they always link to the wrong place

.changeset/sixty-days-think.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: correctly link to assets inlined by the `inlineStyleThreshold` option

packages/kit/src/exports/vite/build/build_server.js

@@ -3,6 +3,7 @@ import { mkdirp } from '../../../utils/filesystem.js';
 import { find_deps, resolve_symlinks } from './utils.js';
 import { s } from '../../../utils/misc.js';
 import { normalizePath } from 'vite';
+import { basename } from 'node:path';
 
 /**
  * @param {string} out
@@ -17,18 +18,47 @@ export function build_server_nodes(out, kit, manifest_data, server_manifest, cli
 	mkdirp(`${out}/server/nodes`);
 	mkdirp(`${out}/server/stylesheets`);
 
+	/** @type {Map<string, string>} */
 	const stylesheet_lookup = new Map();
 
 	if (css) {
-		css.forEach((asset) => {
-			if (asset.source.length < kit.inlineStyleThreshold) {
-				const index = stylesheet_lookup.size;
-				const file = `${out}/server/stylesheets/${index}.js`;
+		/** @type {Set<string>} */
+		const client_stylesheets = new Set();
+		for (const key in client_manifest) {
+			const file = client_manifest[key];
+			if (file.css?.[0]) {
+				client_stylesheets.add(file.css[0]);
+			}
+		}
+
+		/** @type {Map<number, string>} */
+		const server_stylesheets = new Map();
 
-				fs.writeFileSync(file, `// ${asset.fileName}\nexport default ${s(asset.source)};`);
-				stylesheet_lookup.set(asset.fileName, index);
+		const component_stylesheet_map = new Map(Object.values(server_manifest).map((file) => [file.src, file.css?.[0]]));
+
+		manifest_data.nodes.forEach((node, i) => {
+			const server_stylesheet = component_stylesheet_map.get(node.component);
+			if (node.component && server_stylesheet) {
+				server_stylesheets.set(i, server_stylesheet);
 			}
 		});
+
+		// ignore dynamically imported stylesheets since we can't inline those
+		css.filter(asset => client_stylesheets.has(asset.fileName))
+			.forEach((asset) => {
+				if (asset.source.length < kit.inlineStyleThreshold) {
+					const [index] = basename(asset.fileName).split('.');
+					const server_stylesheet = server_stylesheets.get(+index);
+					const file = `${out}/server/stylesheets/${index}.js`;
+
+					// we need to inline the server stylesheet instead of the client one
+					// so that asset paths are correct on document load
+					const source = fs.readFileSync(`${out}/server/${server_stylesheet}`, 'utf-8');
+
+					fs.writeFileSync(file, `// ${server_stylesheet}\nexport default ${s(source)};`);
+					stylesheet_lookup.set(asset.fileName, index);
+				}
+			});
 	}
 
 	manifest_data.nodes.forEach((node, i) => {

packages/kit/test/apps/options/package.json

@@ -13,6 +13,7 @@
 		"test:build": "playwright test"
 	},
 	"devDependencies": {
+		"@fontsource/libre-barcode-128-text": "^5.1.0",
 		"@sveltejs/kit": "workspace:^",
 		"@sveltejs/vite-plugin-svelte": "^5.0.1",
 		"cross-env": "^7.0.3",

packages/kit/test/apps/options/source/pages/inline-assets/+page.svelte

@@ -0,0 +1,11 @@
+<script>
+	import '@fontsource/libre-barcode-128-text';
+</script>
+
+<p>Hello world!</p>
+
+<style>
+	p {
+		font-family: 'Libre Barcode 128 Text', sans-serif;
+	}
+</style>

packages/kit/test/apps/options/test/test.js

@@ -303,6 +303,22 @@ if (!process.env.DEV) {
 			expect(await page.content()).not.toMatch('navigator.serviceWorker');
 		});
 	});
+
+	test.describe('inlineStyleThreshold', () => {
+		test('loads asset', async ({ page }) => {
+			let fontLoaded = false;
+
+			page.on('response', (response) => {
+				if (response.url().endsWith('.woff2') || response.url().endsWith('.woff')) {
+					fontLoaded = response.ok();
+				}
+			});
+
+			await page.goto('/path-base/inline-assets');
+
+			expect(fontLoaded).toBeTruthy();
+		});
+	});
 }
 
 test.describe('Vite options', () => {