fix: ensure bound input content is resumed on hydration (#11986)

* fix: ensure bound input content is resumed on hydration

* fix: ensure bound input content is resumed on hydration

* Update packages/svelte/src/internal/client/dom/elements/bindings/input.js

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

* fix: ensure bound input content is resumed on hydration

* fix: ensure bound input content is resumed on hydration

* Update packages/svelte/src/internal/client/dom/elements/bindings/input.js

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

* add test

* add test

* add test

* add test

* Update packages/svelte/src/internal/client/dom/elements/bindings/input.js

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

* add test

* add test

* newlines between multi-line blocks, let the code breathe

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/flat-feet-visit.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: ensure bound input content is resumed on hydration

packages/svelte/src/internal/client/dom/elements/bindings/input.js

@@ -4,6 +4,7 @@ import { listen_to_event_and_reset_event } from './shared.js';
 import * as e from '../../../errors.js';
 import { get_proxied_value, is } from '../../../proxy.js';
 import { queue_micro_task } from '../../task.js';
+import { hydrating } from '../../hydration.js';
 
 /**
  * @param {HTMLInputElement} input
@@ -29,8 +30,12 @@ export function bind_value(input, get_value, update) {
 
 		var value = get_value();
 
-		// @ts-ignore
+		// If we are hydrating and the value has since changed, then use the update value
-		input.__value = value;
+		// from the input instead.
+		if (hydrating && input.defaultValue !== input.value) {
+			update(input.value);
+			return;
+		}
 
 		if (is_numberlike_input(input) && value === to_number(input.value)) {
 			// handles 0 vs 00 case (see https://github.com/sveltejs/svelte/issues/9959)
@@ -60,6 +65,9 @@ export function bind_group(inputs, group_index, input, get_value, update) {
 	var is_checkbox = input.getAttribute('type') === 'checkbox';
 	var binding_group = inputs;
 
+	// needs to be let or related code isn't treeshaken out if it's always false
+	let hydration_mismatch = false;
+
 	if (group_index !== null) {
 		for (var index of group_index) {
 			var group = binding_group;
@@ -94,6 +102,13 @@ export function bind_group(inputs, group_index, input, get_value, update) {
 	render_effect(() => {
 		var value = get_value();
 
+		// If we are hydrating and the value has since changed, then use the update value
+		// from the input instead.
+		if (hydrating && input.defaultChecked !== input.checked) {
+			hydration_mismatch = true;
+			return;
+		}
+
 		if (is_checkbox) {
 			value = value || [];
 			// @ts-ignore
@@ -115,6 +130,20 @@ export function bind_group(inputs, group_index, input, get_value, update) {
 	queue_micro_task(() => {
 		// necessary to maintain binding group order in all insertion scenarios. TODO optimise
 		binding_group.sort((a, b) => (a.compareDocumentPosition(b) === 4 ? -1 : 1));
+
+		if (hydration_mismatch) {
+			var value;
+
+			if (is_checkbox) {
+				value = get_binding_group_value(binding_group, value, input.checked);
+			} else {
+				var hydration_input = binding_group.find((input) => input.checked);
+				// @ts-ignore
+				value = hydration_input?.__value;
+			}
+
+			update(value);
+		}
 	});
 }
 

packages/svelte/tests/runtime-legacy/shared.ts

@@ -59,6 +59,7 @@ export interface RuntimeTest<Props extends Record<string, any> = Record<string,
 		};
 		logs: any[];
 		warnings: any[];
+		hydrate: Function;
 	}) => void | Promise<void>;
 	test_ssr?: (args: { assert: Assert }) => void | Promise<void>;
 	accessors?: boolean;
@@ -103,6 +104,10 @@ export function runtime_suite(runes: boolean) {
 				if (config.skip_mode?.includes('hydrate')) return true;
 			}
 
+			if (variant === 'dom' && config.skip_mode?.includes('client')) {
+				return 'no-test';
+			}
+
 			if (variant === 'ssr') {
 				if (
 					(config.mode && !config.mode.includes('server')) ||
@@ -161,6 +166,7 @@ async function run_test_variant(
 
 	let logs: string[] = [];
 	let warnings: string[] = [];
+	let manual_hydrate = false;
 
 	{
 		// use some crude static analysis to determine if logs/warnings are intercepted.
@@ -180,6 +186,10 @@ async function run_test_variant(
 			console.log = (...args) => logs.push(...args);
 		}
 
+		if (str.slice(0, i).includes('hydrate')) {
+			manual_hydrate = true;
+		}
+
 		if (str.slice(0, i).includes('warnings') || config.warnings) {
 			// eslint-disable-next-line no-console
 			console.warn = (...args) => {
@@ -297,17 +307,30 @@ async function run_test_variant(
 
 			let instance: any;
 			let props: any;
+			let hydrate_fn: Function = () => {
+				throw new Error('Ensure dom mode is skipped');
+			};
 
 			if (runes) {
 				props = proxy({ ...(config.props || {}) });
-
+				if (manual_hydrate) {
-				const render = variant === 'hydrate' ? hydrate : mount;
+					hydrate_fn = () => {
-				instance = render(mod.default, {
+						instance = hydrate(mod.default, {
-					target,
+							target,
-					props,
+							props,
-					intro: config.intro,
+							intro: config.intro,
-					recover: config.recover ?? false
+							recover: config.recover ?? false
-				});
+						});
+					};
+				} else {
+					const render = variant === 'hydrate' ? hydrate : mount;
+					instance = render(mod.default, {
+						target,
+						props,
+						intro: config.intro,
+						recover: config.recover ?? false
+					});
+				}
 			} else {
 				instance = createClassComponent({
 					component: mod.default,
@@ -357,7 +380,8 @@ async function run_test_variant(
 						raf,
 						compileOptions,
 						logs,
-						warnings
+						warnings,
+						hydrate: hydrate_fn
 					});
 				}
 

packages/svelte/tests/runtime-runes/samples/hydrate-modified-input-group/_config.js

@@ -0,0 +1,18 @@
+import { test } from '../../test';
+
+export default test({
+	skip_mode: ['client'],
+
+	test({ assert, target, hydrate }) {
+		const inputs = /** @type {NodeListOf<HTMLInputElement>} */ (target.querySelectorAll('input'));
+		inputs[1].checked = true;
+		inputs[1].dispatchEvent(new window.Event('change'));
+		// Hydration shouldn't reset the value to 1
+		hydrate();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			'<input name="foo" type="radio" value="1"><input name="foo" type="radio" value="2"><input name="foo" type="radio" value="3">\n2'
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/hydrate-modified-input-group/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	let value = $state(1);
+</script>
+
+{#each [1, 2, 3] as number}
+	<input type="radio" name="foo" value={number} bind:group={value}>
+{/each}
+
+{value}

packages/svelte/tests/runtime-runes/samples/hydrate-modified-input/_config.js

@@ -0,0 +1,15 @@
+import { test } from '../../test';
+
+export default test({
+	skip_mode: ['client'],
+
+	test({ assert, target, hydrate }) {
+		const input = /** @type {HTMLInputElement} */ (target.querySelector('input'));
+		input.value = 'foo';
+		input.dispatchEvent(new window.Event('input'));
+		// Hydration shouldn't reset the value to empty
+		hydrate();
+
+		assert.htmlEqual(target.innerHTML, '<input type="text">\nfoo');
+	}
+});

packages/svelte/tests/runtime-runes/samples/hydrate-modified-input/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	let value = $state('');
+</script>
+
+<input type="text" bind:value={value}>
+{value}

playgrounds/demo/src/entry-client.ts

@@ -4,8 +4,11 @@ import App from './App.svelte';
 const root = document.getElementById('root')!;
 const render = root.firstChild?.nextSibling ? hydrate : mount;
 
-const component = render(App, {
+setTimeout(() => {
-	target: document.getElementById('root')!
+	const component = render(App, {
-});
+		target: document.getElementById('root')!
-// @ts-ignore
+	});
-window.unmount = () => unmount(component);
+	// @ts-ignore
+	window.unmount = () => unmount(component);
+}, 2000)
+