fix: keep input in sync when binding updated via effect

Fixes #16413

The previous fix was insufficient as it didn't account for effects running as a result of a change, which is executed in a different batch. Instead we now set a boolean that is true while an async branch is flushed.

.changeset/grumpy-boats-beg.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: keep input in sync when binding updated via effect

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

@@ -1,4 +1,3 @@
-/** @import { Batch } from '../../../reactivity/batch.js' */
 import { DEV } from 'esm-env';
 import { render_effect, teardown } from '../../../reactivity/effects.js';
 import { listen_to_event_and_reset_event } from './shared.js';
@@ -19,8 +18,6 @@ import { current_batch } from '../../../reactivity/batch.js';
 export function bind_value(input, get, set = get) {
 	var runes = is_runes();
 
-	var batches = new WeakSet();
-
 	listen_to_event_and_reset_event(input, 'input', (is_reset) => {
 		if (DEV && input.type === 'checkbox') {
 			// TODO should this happen in prod too?
@@ -32,10 +29,6 @@ export function bind_value(input, get, set = get) {
 		value = is_numberlike_input(input) ? to_number(value) : value;
 		set(value);
 
-		if (current_batch !== null) {
-			batches.add(current_batch);
-		}
-
 		// In runes mode, respect any validation in accessors (doesn't apply in legacy mode,
 		// because we use mutable state which ensures the render effect always runs)
 		if (runes && value !== (value = get())) {
@@ -62,10 +55,6 @@ export function bind_value(input, get, set = get) {
 		(untrack(get) == null && input.value)
 	) {
 		set(is_numberlike_input(input) ? to_number(input.value) : input.value);
-
-		if (current_batch !== null) {
-			batches.add(current_batch);
-		}
 	}
 
 	render_effect(() => {
@@ -76,9 +65,9 @@ export function bind_value(input, get, set = get) {
 
 		var value = get();
 
-		if (input === document.activeElement && batches.has(/** @type {Batch} */ (current_batch))) {
-			// Never rewrite the contents of a focused input. We can get here if, for example,
-			// an update is deferred because of async work depending on the input:
+		if (input === document.activeElement && current_batch?.flushing_async) {
+			// Never rewrite the contents of a focused input when flushing async work.
+			// We can get here if, for example, an update is deferred because of async work depending on the input:
 			//
 			// <input bind:value={query}>
 			// <p>{await find(query)}</p>

packages/svelte/src/internal/client/reactivity/batch.js

@@ -152,6 +152,11 @@ export class Batch {
 	 */
 	skipped_effects = new Set();
 
+	/**
+	 * True while a batch that had asynchronous work (i.e. a pending count) is being flushed.
+	 */
+	flushing_async = false;
+
 	/**
 	 *
 	 * @param {Effect[]} root_effects
@@ -412,6 +417,8 @@ export class Batch {
 		this.#pending -= 1;
 
 		if (this.#pending === 0) {
+			this.flushing_async = true;
+
 			for (const e of this.#render_effects) {
 				set_signal_status(e, DIRTY);
 				schedule_effect(e);
@@ -431,6 +438,8 @@ export class Batch {
 			this.#effects = [];
 
 			this.flush();
+
+			this.flushing_async = true;
 		} else {
 			this.deactivate();
 		}

packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused/_config.js

@@ -0,0 +1,37 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await new Promise((resolve) => setTimeout(resolve, 110));
+
+		const [input] = target.querySelectorAll('input');
+
+		assert.equal(input.value, 'a');
+		assert.htmlEqual(target.innerHTML, `<p>a</p><input />`);
+
+		flushSync(() => {
+			input.focus();
+			input.value = 'ab';
+			input.dispatchEvent(new InputEvent('input', { bubbles: true }));
+		});
+
+		await new Promise((resolve) => setTimeout(resolve, 50));
+
+		flushSync(() => {
+			input.focus();
+			input.value = 'abc';
+			input.dispatchEvent(new InputEvent('input', { bubbles: true }));
+		});
+
+		await new Promise((resolve) => setTimeout(resolve, 60));
+
+		assert.equal(input.value, 'abc');
+		assert.htmlEqual(target.innerHTML, `<p>ab</p><input />`);
+
+		await new Promise((resolve) => setTimeout(resolve, 60));
+
+		assert.equal(input.value, 'abc');
+		assert.htmlEqual(target.innerHTML, `<p>abc</p><input />`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	let value = $state('a');
+
+	function push(value) {
+		// Cannot use a queue and flush it manually here, because we need the input to be focused
+		const deferred = Promise.withResolvers();
+		setTimeout(() => deferred.resolve(value), 100);
+		return deferred.promise;
+	}
+</script>
+
+<svelte:boundary>
+	<p>{await push(value)}</p>
+	<input bind:value />
+
+	{#snippet pending()}
+		<p>loading</p>
+	{/snippet}
+</svelte:boundary>

packages/svelte/tests/runtime-runes/samples/binding-update-while-focused-2/_config.js

@@ -0,0 +1,27 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [input] = target.querySelectorAll('input');
+
+		assert.equal(input.value, '2');
+		assert.htmlEqual(target.innerHTML, `<p>2</p><input type="number" />`);
+
+		flushSync(() => {
+			input.focus();
+			input.value = '3';
+			input.dispatchEvent(new InputEvent('input', { bubbles: true }));
+		});
+		assert.equal(input.value, '3');
+		assert.htmlEqual(target.innerHTML, `<p>3</p><input type="number" />`);
+
+		flushSync(() => {
+			input.focus();
+			input.value = '6';
+			input.dispatchEvent(new InputEvent('input', { bubbles: true }));
+		});
+		assert.equal(input.value, '5');
+		assert.htmlEqual(target.innerHTML, `<p>5</p><input type="number" />`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/binding-update-while-focused-2/main.svelte

@@ -0,0 +1,22 @@
+<script>
+	let value = $state(0)
+
+	const min = 2
+	const max = 5
+
+	$effect(() => {
+		setValue()
+	})
+
+	function setValue() {
+		if (value < min) {
+			value = min
+		}
+		if (value > max) {
+			value = max
+		}
+	}
+</script>
+
+<p>{value}</p>
+<input type="number" bind:value />