feat: allow to control behavior on hydration error

.changeset/orange-apes-change.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: allow to control behavior on hydration error

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

@@ -96,7 +96,8 @@ const get_defaults = (prefix = '') => ({
 		outDir: join(prefix, '.svelte-kit'),
 		router: {
 			type: 'pathname',
-			resolution: 'client'
+			resolution: 'client',
+			hydrationErrorHandling: 'error'
 		},
 		serviceWorker: {
 			register: true

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

@@ -262,7 +262,8 @@ const options = object(
 
 			router: object({
 				type: list(['pathname', 'hash']),
-				resolution: list(['client', 'server'])
+				resolution: list(['client', 'server']),
+				hydrationErrorHandling: list(['error', 'keep html'])
 			}),
 
 			serviceWorker: object({

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

@@ -681,6 +681,16 @@ export interface KitConfig {
 		 * @since 2.17.0
 		 */
 		resolution?: 'client' | 'server';
+		/**
+		 * Sometimes the server renders the HTML successfully, but during hydration on the client something goes wrong. One example is a flaky network where
+		 * the request for one of the JavaScript files that is needed for a page to work fails. This option allows you to configure what should happen in such a case:
+		 * - `'error'` (default) - the client will fall back to the root error page. The user will immediately see something's off and depending on the error page can act accordingly. Use this if your app is too sensitive to not-immediately-visible broken states.
+		 * - `'keep html'` - the client will show the HTML that was rendered on the server and not attempt to hydrate the page. The user will see the successful server-rendered page, but no interactivity will be available and navigations are full page reloads. Use this if your app can largely function without JavaScript.
+		 *
+		 * @default "error"
+		 * @since 2.18.0
+		 */
+		hydrationErrorHandling?: 'error' | 'keep html';
 	};
 	serviceWorker?: {
 		/**

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

@@ -321,7 +321,9 @@ async function kit({ svelte_config }) {
 					__SVELTEKIT_APP_VERSION_POLL_INTERVAL__: s(kit.version.pollInterval),
 					__SVELTEKIT_DEV__: 'false',
 					__SVELTEKIT_EMBEDDED__: kit.embedded ? 'true' : 'false',
-					__SVELTEKIT_CLIENT_ROUTING__: kit.router.resolution === 'client' ? 'true' : 'false'
+					__SVELTEKIT_CLIENT_ROUTING__: kit.router.resolution === 'client' ? 'true' : 'false',
+					__SVELTEKIT_NO_ROOT_ERROR_ON_HYDRATION__:
+						kit.router.hydrationErrorHandling === 'keep html'
 				};
 
 				if (!secondary_build_started) {
@@ -332,7 +334,9 @@ async function kit({ svelte_config }) {
 					__SVELTEKIT_APP_VERSION_POLL_INTERVAL__: '0',
 					__SVELTEKIT_DEV__: 'true',
 					__SVELTEKIT_EMBEDDED__: kit.embedded ? 'true' : 'false',
-					__SVELTEKIT_CLIENT_ROUTING__: kit.router.resolution === 'client' ? 'true' : 'false'
+					__SVELTEKIT_CLIENT_ROUTING__: kit.router.resolution === 'client' ? 'true' : 'false',
+					__SVELTEKIT_NO_ROOT_ERROR_ON_HYDRATION__:
+						kit.router.hydrationErrorHandling === 'keep html'
 				};
 
 				// These Kit dependencies are packaged as CommonJS, which means they must always be externalized.

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

@@ -212,6 +212,7 @@ let current = {
 /** this being true means we SSR'd */
 let hydrated = false;
 let started = false;
+let errored_on_hydration = false;
 let autoscroll = true;
 let updating = false;
 let is_navigating = false;
@@ -1242,7 +1243,7 @@ function get_rerouted_url(url) {
  * @returns {Promise<import('./types.js').NavigationIntent | undefined>}
  */
 async function get_navigation_intent(url, invalidating) {
-	if (!url) return;
+	if (!url || errored_on_hydration) return;
 	if (is_external_url(url, base, app.hash)) return;
 
 	if (__SVELTEKIT_CLIENT_ROUTING__) {
@@ -1678,7 +1679,7 @@ function setup_preload() {
 	 */
 	async function preload(element, priority) {
 		const a = find_anchor(element, container);
-		if (!a || a === current_a) return;
+		if (!a || a === current_a || errored_on_hydration) return;
 
 		const { url, external, download } = get_link_info(a, base, app.hash);
 		if (external || download) return;
@@ -2213,7 +2214,7 @@ function _start_router() {
 		if (event.defaultPrevented) return;
 
 		const a = find_anchor(/** @type {Element} */ (event.composedPath()[0]), container);
-		if (!a) return;
+		if (!a || errored_on_hydration) return;
 
 		const { url, external, target, download } = get_link_info(a, base, app.hash);
 		if (!url) return;
@@ -2584,15 +2585,20 @@ async function _hydrate(
 			return;
 		}
 
-		result = await load_root_error_page({
-			status: get_status(error),
-			error: await handle_error(error, { url, params, route }),
-			url,
-			route
-		});
+		if (__SVELTEKIT_NO_ROOT_ERROR_ON_HYDRATION__) {
+			errored_on_hydration = true;
+			return;
+		} else {
+			result = await load_root_error_page({
+				status: get_status(error),
+				error: await handle_error(error, { url, params, route }),
+				url,
+				route
+			});
 
-		target.textContent = '';
-		hydrate = false;
+			target.textContent = '';
+			hydrate = false;
+		}
 	}
 
 	if (result.props.page) {

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

@@ -6,6 +6,8 @@ declare global {
 	const __SVELTEKIT_EMBEDDED__: boolean;
 	/** True if `config.kit.router.resolution === 'client'` */
 	const __SVELTEKIT_CLIENT_ROUTING__: boolean;
+	/** True if `config.kit.router.hydrationErrorHandling === 'keep html'` */
+	const __SVELTEKIT_NO_ROOT_ERROR_ON_HYDRATION__: boolean;
 	/**
 	 * This makes the use of specific features visible at both dev and build time, in such a
 	 * way that we can error when they are not supported by the target platform.

packages/kit/types/index.d.ts

@@ -663,6 +663,16 @@ declare module '@sveltejs/kit' {
 			 * @since 2.17.0
 			 */
 			resolution?: 'client' | 'server';
+			/**
+			 * Sometimes the server renders the HTML successfully, but during hydration on the client something goes wrong. One example is a flaky network where
+			 * the request for one of the JavaScript files that is needed for a page to work fails. This option allows you to configure what should happen in such a case:
+			 * - `'error'` (default) - the client will fall back to the root error page. The user will immediately see something's off and depending on the error page can act accordingly. Use this if your app is too sensitive to not-immediately-visible broken states.
+			 * - `'keep html'` - the client will show the HTML that was rendered on the server and not attempt to hydrate the page. The user will see the successful server-rendered page, but no interactivity will be available and navigations are full page reloads. Use this if your app can largely function without JavaScript.
+			 *
+			 * @default "error"
+			 * @since 2.18.0
+			 */
+			hydrationErrorHandling?: 'error' | 'keep html';
 		};
 		serviceWorker?: {
 			/**