fix: add experimental.forkPreloads flag

.changeset/tame-lights-push.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: put forking behind `experimental.forkPreloads`

packages/kit/src/core/config/index.spec.js

@@ -80,7 +80,8 @@ const get_defaults = (prefix = '') => ({
 		experimental: {
 			tracing: { server: false },
 			instrumentation: { server: false },
-			remoteFunctions: false
+			remoteFunctions: false,
+			forkPreloads: false
 		},
 		files: {
 			src: join(prefix, 'src'),
@@ -479,6 +480,30 @@ test('errors on invalid tracing values', () => {
 	}, /^config\.kit\.experimental\.tracing\.server should be true or false, if specified$/);
 });
 
+test('errors on invalid forkPreloads values', () => {
+	assert.throws(() => {
+		validate_config({
+			kit: {
+				experimental: {
+					// @ts-expect-error - given value expected to throw
+					forkPreloads: 'true'
+				}
+			}
+		});
+	}, /^config\.kit\.experimental\.forkPreloads should be true or false, if specified$/);
+
+	assert.throws(() => {
+		validate_config({
+			kit: {
+				experimental: {
+					// @ts-expect-error - given value expected to throw
+					forkPreloads: 1
+				}
+			}
+		});
+	}, /^config\.kit\.experimental\.forkPreloads should be true or false, if specified$/);
+});
+
 test('uses src prefix for other kit.files options', async () => {
 	const cwd = join(__dirname, 'fixtures/custom-src');
 

packages/kit/src/core/config/options.js

@@ -132,7 +132,8 @@ const options = object(
 				instrumentation: object({
 					server: boolean(false)
 				}),
-				remoteFunctions: boolean(false)
+				remoteFunctions: boolean(false),
+				forkPreloads: boolean(false)
 			}),
 
 			files: object({

packages/kit/src/exports/public.d.ts

@@ -505,6 +505,12 @@ export interface KitConfig {
 		 * @default false
 		 */
 		remoteFunctions?: boolean;
+
+		/**
+		 * Whether to enable the experimental forked preloading feature using Svelte's fork API.
+		 * @default false
+		 */
+		forkPreloads?: boolean;
 	};
 	/**
 	 * Where to find various files within your project.

packages/kit/src/exports/vite/index.js

@@ -346,6 +346,7 @@ async function kit({ svelte_config }) {
 				__SVELTEKIT_APP_DIR__: s(kit.appDir),
 				__SVELTEKIT_EMBEDDED__: s(kit.embedded),
 				__SVELTEKIT_EXPERIMENTAL__REMOTE_FUNCTIONS__: s(kit.experimental.remoteFunctions),
+				__SVELTEKIT_FORK_PRELOADS__: s(kit.experimental.forkPreloads),
 				__SVELTEKIT_PATHS_ASSETS__: s(kit.paths.assets),
 				__SVELTEKIT_PATHS_BASE__: s(kit.paths.base),
 				__SVELTEKIT_PATHS_RELATIVE__: s(kit.paths.relative),

packages/kit/src/runtime/client/client.js

@@ -532,7 +532,7 @@ async function _preload_data(intent) {
 			fork: null
 		};
 
-		if (svelte.fork) {
+		if (__SVELTEKIT_FORK_PRELOADS__ && svelte.fork) {
 			const lc = load_cache;
 
 			lc.fork = lc.promise.then((result) => {
@@ -545,7 +545,7 @@ async function _preload_data(intent) {
 							update(result.props.page);
 						});
 					} catch {
-						// if it errors, it's because the experimental flag isn't enabled
+						// if it errors, it's because the experimental flag isn't enabled in Svelte
 					}
 				}
 

packages/kit/src/types/global-private.d.ts

@@ -11,6 +11,8 @@ declare global {
 	const __SVELTEKIT_SERVER_TRACING_ENABLED__: boolean;
 	/** true if corresponding config option is set to true */
 	const __SVELTEKIT_EXPERIMENTAL__REMOTE_FUNCTIONS__: boolean;
+	/** True if `config.kit.experimental.forkPreloads` is `true` */
+	const __SVELTEKIT_FORK_PRELOADS__: boolean;
 	/** True if `config.kit.router.resolution === 'client'` */
 	const __SVELTEKIT_CLIENT_ROUTING__: boolean;
 	/** True if `config.kit.router.type === 'hash'` */

packages/kit/types/index.d.ts

@@ -481,6 +481,12 @@ declare module '@sveltejs/kit' {
 			 * @default false
 			 */
 			remoteFunctions?: boolean;
+
+			/**
+			 * Whether to enable the experimental forked preloading feature using Svelte's fork API.
+			 * @default false
+			 */
+			forkPreloads?: boolean;
 		};
 		/**
 		 * Where to find various files within your project.