fix: don't update a focused input with values from its own past

.changeset/fast-mails-fail.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't update a focused input with values from its own past

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

@@ -8,7 +8,7 @@ import { queue_micro_task } from '../../task.js';
 import { hydrating } from '../../hydration.js';
 import { untrack } from '../../../runtime.js';
 import { is_runes } from '../../../context.js';
-import { current_batch } from '../../../reactivity/batch.js';
+import { current_batch, previous_batch } from '../../../reactivity/batch.js';
 
 /**
  * @param {HTMLInputElement} input
@@ -76,7 +76,7 @@ export function bind_value(input, get, set = get) {
 
 		var value = get();
 
-		if (input === document.activeElement && batches.has(/** @type {Batch} */ (current_batch))) {
+		if (input === document.activeElement && batches.has(/** @type {Batch} */ (previous_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:
 			//

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

@@ -38,6 +38,9 @@ const batches = new Set();
 /** @type {Batch | null} */
 export let current_batch = null;
 
+/** @type {Batch | null} */
+export let previous_batch = current_batch;
+
 /**
  * When time travelling, we re-evaluate deriveds based on the temporary
  * values of their dependencies rather than their actual values, and cache
@@ -72,7 +75,10 @@ let is_flushing = false;
 
 let is_flushing_sync = false;
 
+let uid = 1;
 export class Batch {
+	id = uid++;
+
 	/**
 	 * The current values of any sources that are updated in this batch
 	 * They keys of this map are identical to `this.#previous`
@@ -218,6 +224,7 @@ export class Batch {
 
 			// If sources are written to, then work needs to happen in a separate batch, else prior sources would be mixed with
 			// newly updated sources, which could lead to infinite loops when effects run over and over again.
+			previous_batch = current_batch;
 			current_batch = null;
 
 			flush_queued_effects(render_effects);
@@ -350,6 +357,7 @@ export class Batch {
 
 	deactivate() {
 		current_batch = null;
+		previous_batch = null;
 
 		for (const update of effect_pending_updates) {
 			effect_pending_updates.delete(update);

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

@@ -0,0 +1,37 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, instance }) {
+		instance.shift();
+		await tick();
+
+		const [input] = target.querySelectorAll('input');
+
+		input.focus();
+		input.value = '1';
+		input.dispatchEvent(new InputEvent('input', { bubbles: true }));
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, `<input type="number" /> <p>0</p>`);
+		assert.equal(input.value, '1');
+
+		input.focus();
+		input.value = '2';
+		input.dispatchEvent(new InputEvent('input', { bubbles: true }));
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, `<input type="number" /> <p>0</p>`);
+		assert.equal(input.value, '2');
+
+		instance.shift();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<input type="number" /> <p>1</p>`);
+		assert.equal(input.value, '2');
+
+		instance.shift();
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<input type="number" /> <p>2</p>`);
+		assert.equal(input.value, '2');
+	}
+});

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

@@ -0,0 +1,25 @@
+<script lang="ts">
+	let count = $state(0);
+
+	let deferreds = [];
+
+	export function shift() {
+		const d = deferreds.shift();
+		d.d.resolve(d.v);
+	}
+
+	function push(v) {
+		const d = Promise.withResolvers();
+		deferreds.push({ d, v });
+		return d.promise;
+	}
+</script>
+
+<svelte:boundary>
+	<input type="number" bind:value={count} />
+	<p>{await push(count)}</p>
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>