fix: preserve `<select>` state while focused (#16958)

.changeset/quiet-weeks-camp.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: preserve `<select>` state while focused

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

@@ -3,6 +3,7 @@ import { listen_to_event_and_reset_event } from './shared.js';
 import { is } from '../../../proxy.js';
 import { is_array } from '../../../../shared/utils.js';
 import * as w from '../../../warnings.js';
+import { Batch, current_batch, previous_batch } from '../../../reactivity/batch.js';
 
 /**
  * Selects the correct option(s) (depending on whether this is a multiple select)
@@ -83,6 +84,7 @@ export function init_select(select) {
  * @returns {void}
  */
 export function bind_select_value(select, get, set = get) {
+	var batches = new WeakSet();
 	var mounting = true;
 
 	listen_to_event_and_reset_event(select, 'change', (is_reset) => {
@@ -102,11 +104,30 @@ export function bind_select_value(select, get, set = get) {
 		}
 
 		set(value);
+
+		if (current_batch !== null) {
+			batches.add(current_batch);
+		}
 	});
 
 	// Needs to be an effect, not a render_effect, so that in case of each loops the logic runs after the each block has updated
 	effect(() => {
 		var value = get();
+
+		if (select === document.activeElement) {
+			// we need both, because in non-async mode, render effects run before previous_batch is set
+			var batch = /** @type {Batch} */ (previous_batch ?? current_batch);
+
+			// Don't update the <select> if it is focused. We can get here if, for example,
+			// an update is deferred because of async work depending on the select:
+			//
+			// <select bind:value={selected}>...</select>
+			// <p>{await find(selected)}</p>
+			if (batches.has(batch)) {
+				return;
+			}
+		}
+
 		select_option(select, value, mounting);
 
 		// Mounting and value undefined -> take selection from dom

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

@@ -2,8 +2,9 @@ import { tick } from 'svelte';
 import { test } from '../../test';
 
 export default test({
-	async test({ assert, target, instance }) {
-		instance.shift();
+	async test({ assert, target }) {
+		const [shift] = target.querySelectorAll('button');
+		shift.click();
 		await tick();
 
 		const [input] = target.querySelectorAll('input');
@@ -13,7 +14,7 @@ export default test({
 		input.dispatchEvent(new InputEvent('input', { bubbles: true }));
 		await tick();
 
-		assert.htmlEqual(target.innerHTML, `<input type="number" /> <p>0</p>`);
+		assert.htmlEqual(target.innerHTML, `<button>shift</button><input type="number" /> <p>0</p>`);
 		assert.equal(input.value, '1');
 
 		input.focus();
@@ -21,17 +22,17 @@ export default test({
 		input.dispatchEvent(new InputEvent('input', { bubbles: true }));
 		await tick();
 
-		assert.htmlEqual(target.innerHTML, `<input type="number" /> <p>0</p>`);
+		assert.htmlEqual(target.innerHTML, `<button>shift</button><input type="number" /> <p>0</p>`);
 		assert.equal(input.value, '2');
 
-		instance.shift();
+		shift.click();
 		await tick();
-		assert.htmlEqual(target.innerHTML, `<input type="number" /> <p>1</p>`);
+		assert.htmlEqual(target.innerHTML, `<button>shift</button><input type="number" /> <p>1</p>`);
 		assert.equal(input.value, '2');
 
-		instance.shift();
+		shift.click();
 		await tick();
-		assert.htmlEqual(target.innerHTML, `<input type="number" /> <p>2</p>`);
+		assert.htmlEqual(target.innerHTML, `<button>shift</button><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

@@ -1,22 +1,23 @@
 <script lang="ts">
 	let count = $state(0);
 
-	let deferreds = [];
+	let resolvers = [];
+	let input;
 
-	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;
+	function push(value) {
+		const { promise, resolve } = Promise.withResolvers();
+		resolvers.push(() => resolve(value));
+		return promise;
 	}
 </script>
 
+<button onclick={() => {
+	input.focus();
+	resolvers.shift()?.();
+}}>shift</button>
+
 <svelte:boundary>
-	<input type="number" bind:value={count} />
+	<input bind:this={input} type="number" bind:value={count} />
 	<p>{await push(count)}</p>
 
 	{#snippet pending()}

packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused-3/_config.js

@@ -0,0 +1,82 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [shift] = target.querySelectorAll('button');
+		shift.click();
+		await tick();
+
+		const [select] = target.querySelectorAll('select');
+
+		select.focus();
+		select.value = 'three';
+		select.dispatchEvent(new InputEvent('change', { bubbles: true }));
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<select>
+					<option>one</option>
+					<option>two</option>
+					<option>three</option>
+				</select>
+				<p>two</p>
+			`
+		);
+		assert.equal(select.value, 'three');
+
+		select.focus();
+		select.value = 'one';
+		select.dispatchEvent(new InputEvent('change', { bubbles: true }));
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<select>
+					<option>one</option>
+					<option>two</option>
+					<option>three</option>
+				</select>
+				<p>two</p>
+			`
+		);
+		assert.equal(select.value, 'one');
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<select>
+					<option>one</option>
+					<option>two</option>
+					<option>three</option>
+				</select>
+				<p>three</p>
+			`
+		);
+		assert.equal(select.value, 'one');
+
+		shift.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<select>
+					<option>one</option>
+					<option>two</option>
+					<option>three</option>
+				</select>
+				<p>one</p>
+			`
+		);
+		assert.equal(select.value, 'one');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-binding-update-while-focused-3/main.svelte

@@ -0,0 +1,31 @@
+<script lang="ts">
+	let selected = $state('two');
+
+	let resolvers = [];
+	let select;
+
+	function push(value) {
+		const { promise, resolve } = Promise.withResolvers();
+		resolvers.push(() => resolve(value));
+		return promise;
+	}
+</script>
+
+<button onclick={() => {
+	select.focus();
+	resolvers.shift()?.();
+}}>shift</button>
+
+<svelte:boundary>
+	<select bind:this={select} bind:value={selected}>
+		<option>one</option>
+		<option>two</option>
+		<option>three</option>
+	</select>
+
+	<p>{await push(selected)}</p>
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>