fix: apply `<select multiple defaultValue>` by membership

`set_default_select_value` compared every option against the whole value,
so a `multiple` select never matched and its default selection was lost.
Mirror `select_option` instead: branch on `select.multiple` and test
membership, leaving a non-array value alone.

Restoring `selectedIndex` only preserves a single selection, so the
multiple branch snapshots `selectedOptions` and restores each option,
keeping the current selection untouched.

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -1,7 +1,7 @@
 /** @import { Blocker, Effect } from '#client' */
 import { DEV } from 'esm-env';
 import { hydrating, set_hydrating } from '../hydration.js';
-import { get_descriptors, get_prototype_of } from '../../../shared/utils.js';
+import { get_descriptors, get_prototype_of, is_array } from '../../../shared/utils.js';
 import { create_event, delegate, delegated, event, event_symbol } from './events.js';
 import { add_form_reset_listener, autofocus } from './misc.js';
 import * as w from '../../warnings.js';
@@ -179,6 +179,27 @@ export function set_default_select_value(select, value) {
 	var has_explicit_value = '__value' in select;
 	var selected_index = select.selectedIndex;
 
+	if (select.multiple) {
+		// a `multiple` select takes a list of values, so an option is part of the
+		// default selection when it is a member of that list — mirroring how
+		// `select_option` applies the current value
+		if (!is_array(value)) return;
+
+		var selected = new Set([...select.selectedOptions]);
+
+		for (var multi_option of select.options) {
+			set_selected(multi_option, value.includes(get_option_value(multi_option)));
+		}
+
+		if (has_explicit_value) {
+			for (var option_to_restore of select.options) {
+				option_to_restore.selected = selected.has(option_to_restore);
+			}
+		}
+
+		return;
+	}
+
 	for (var option of select.options) {
 		set_selected(option, is(get_option_value(option), value));
 	}

packages/svelte/tests/runtime-runes/samples/form-default-value-select-multiple/_config.js

@@ -0,0 +1,55 @@
+import { test } from '../../test';
+import { flushSync } from 'svelte';
+
+export default test({
+	async test({ assert, target }) {
+		/**
+		 * @param {NodeListOf<any>} options
+		 * @param {any[]} selected
+		 */
+		function check_options(options, selected) {
+			for (let i = 0; i < options.length; i++) {
+				assert.equal(options[i].selected, selected[i], `option ${i}`);
+			}
+		}
+
+		/**
+		 * @param {HTMLSelectElement} select
+		 * @param {number[]} indexes
+		 */
+		function select_options(select, indexes) {
+			const options = select.querySelectorAll('option');
+			for (let i = 0; i < options.length; i++) {
+				options[i].selected = indexes.includes(i);
+			}
+			select.dispatchEvent(new Event('change', { bubbles: true }));
+		}
+
+		const reset = /** @type {HTMLInputElement} */ (target.querySelector('input[type=reset]'));
+		const [test1, test2] = target.querySelectorAll('select');
+		const span = /** @type {HTMLSpanElement} */ (target.querySelector('span'));
+
+		// every option named by defaultValue is selected, not just none of them
+		check_options(test1.querySelectorAll('option'), [true, false, true]);
+
+		// an explicit value still wins over defaultValue
+		check_options(test2.querySelectorAll('option'), [false, false, true]);
+		assert.htmlEqual(span.innerHTML, 'c');
+
+		// change both selections, then reset the form
+		select_options(test1, [1]);
+		select_options(test2, [0, 1]);
+		flushSync();
+
+		assert.htmlEqual(span.innerHTML, 'a,b');
+
+		reset.click();
+		await Promise.resolve();
+		flushSync();
+
+		// reset restores the default selection in both cases
+		check_options(test1.querySelectorAll('option'), [true, false, true]);
+		check_options(test2.querySelectorAll('option'), [true, false, true]);
+		assert.htmlEqual(span.innerHTML, 'a,c');
+	}
+});

packages/svelte/tests/runtime-runes/samples/form-default-value-select-multiple/main.svelte

@@ -0,0 +1,27 @@
+<script>
+	let selected = $state(['c']);
+
+	let defaultValue = $state(['a', 'c']);
+</script>
+
+<form>
+	<!-- defaultValue=[a, c], no value: the default selection takes effect -->
+	<select multiple {defaultValue}>
+		<option value="a">A</option>
+		<option value="b">B</option>
+		<option value="c">C</option>
+	</select>
+
+	<!-- defaultValue=[a, c], value=[c]: the explicit value wins -->
+	<select multiple {defaultValue} bind:value={selected}>
+		<option value="a">A</option>
+		<option value="b">B</option>
+		<option value="c">C</option>
+	</select>
+
+	<input type="reset" value="Reset" />
+</form>
+
+<p>
+	<span class="test-1">{selected.join(',')}</span>
+</p>