From 864de81bb67372af9783081b039d946dcdb5aabf Mon Sep 17 00:00:00 2001
From: Nic Polumeyv <nicolas.polum@outlook.com>
Date: Wed, 26 Aug 2026 04:10:48 -0400
Subject: [PATCH] fix: keep the current selection of a `<select>` when its
 `defaultValue` is applied (#18719)

Also ensure init_select isn't called multiple times

Follow-up to #18591.
---
 .../client/visitors/RegularElement.js         | 36 ++++++-----
 .../client/dom/elements/attributes.js         |  4 +-
 .../client/dom/elements/bindings/select.js    | 64 ++++++++++++-------
 .../form-default-value-select/_config.js      |  2 +-
 .../_config.js                                | 52 +++++++++++++++
 .../main.svelte                               | 37 +++++++++++
 .../_config.js                                | 30 +++++++++
 .../main.svelte                               | 21 ++++++
 8 files changed, 204 insertions(+), 42 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/select-default-value-preserve-selection/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/select-default-value-preserve-selection/main.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/select-default-value-single-observer/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/select-default-value-single-observer/main.svelte

diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
index 401891b41b..adc9ec72dd 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
@@ -520,20 +520,28 @@ export function RegularElement(node, context) {
 	// deferred from the attribute loop above, so that the options it selects from
 	// have been created and had their values assigned
 	if (!has_spread && name === 'select') {
-		for (const attribute of /** @type {AST.Attribute[]} */ (attributes)) {
-			if (get_attribute_name(node, attribute) === 'defaultValue') {
-				const { value, has_state } = build_attribute_value(attribute.value, context, (v, m) =>
-					context.state.memoizer.add(v, m)
-				);
+		const default_value = /** @type {AST.Attribute[]} */ (attributes).find(
+			(attribute) => get_attribute_name(node, attribute) === 'defaultValue'
+		);
 
-				const update = b.stmt(b.call('$.set_default_select_value', node_id, value));
+		if (default_value) {
+			const { value, has_state } = build_attribute_value(default_value.value, context, (v, m) =>
+				context.state.memoizer.add(v, m)
+			);
 
-				(has_state ? context.state.update : context.state.init).push(update);
-				if (!bindings.has('value')) {
-					context.state.init.push(b.stmt(b.call('$.init_select', node_id)));
-				}
-				break;
-			}
+			(has_state ? context.state.update : context.state.init).push(
+				b.stmt(b.call('$.set_default_select_value', node_id, value))
+			);
+		}
+
+		const value_attribute = lookup.get('value');
+		const dynamic_value =
+			value_attribute !== undefined &&
+			value_attribute.value !== true &&
+			!is_text_attribute(value_attribute);
+
+		if (default_value || dynamic_value || bindings.has('value')) {
+			context.state.init.push(b.stmt(b.call('$.init_select', node_id)));
 		}
 	}
 
@@ -772,10 +780,6 @@ function build_element_special_value_attribute(
 	} else {
 		state.init.push(build_update(value));
 	}
-
-	if (is_select_with_value) {
-		state.init.push(b.stmt(b.call('$.init_select', node_id)));
-	}
 }
 
 /**
diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index 7439db56e9..0fe1b8b2d3 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/packages/svelte/src/internal/client/dom/elements/attributes.js
@@ -525,7 +525,7 @@ export function attribute_effect(
 				var select = /** @type {HTMLSelectElement} */ (element);
 
 				if ('defaultValue' in next) {
-					set_default_select_value(select, next.defaultValue, false);
+					set_default_select_value(select, next.defaultValue);
 				}
 
 				if ('value' in next) {
@@ -558,7 +558,7 @@ export function attribute_effect(
 				var attrs = /** @type {Record<string | symbol, any>} */ (prev);
 
 				if ('defaultValue' in attrs) {
-					set_default_select_value(select, attrs.defaultValue, true);
+					set_default_select_value(select, attrs.defaultValue);
 				}
 
 				select_option(select, attrs.value, true);
diff --git a/packages/svelte/src/internal/client/dom/elements/bindings/select.js b/packages/svelte/src/internal/client/dom/elements/bindings/select.js
index fd1ebb92b8..cac7531ed2 100644
--- a/packages/svelte/src/internal/client/dom/elements/bindings/select.js
+++ b/packages/svelte/src/internal/client/dom/elements/bindings/select.js
@@ -20,33 +20,54 @@ export function set_selected(option, selected) {
 }
 
 /**
- * Sets the options a form reset should restore without changing the current selection.
- * The initial call is allowed to establish the current selection when no value exists.
+ * Sets the options a form reset should restore. The first call selects
+ * them if nothing has set a value, later calls leave the current selection alone.
  * @param {HTMLSelectElement} select
  * @param {any} value
- * @param {boolean} [mounting]
  */
-export function set_default_select_value(select, value, mounting = !('__defaultValue' in select)) {
-	// The DOM cannot recover unmatched, object or multiple defaults from selected options.
-	// Keep the requested value so option mutations can reapply it; property presence also
-	// distinguishes the initial application from later updates when the value is undefined.
+export function set_default_select_value(select, value) {
+	var mounting = !('__defaultValue' in select);
+	// @ts-expect-error
+	if (!mounting && select.__defaultValue === value) return;
 	// @ts-expect-error
 	select.__defaultValue = value;
-	var values = select.multiple ? (value == null ? [] : value) : null;
+	apply_default_select_value(select, !mounting || '__value' in select);
+}
 
-	if (select.multiple && !is_array(values)) return;
-	var selected = !mounting || '__value' in select ? new Set(select.selectedOptions) : null;
+/**
+ * Marks the options matching `__defaultValue` as selected. Without `preserve`
+ * a newly matching option gets selected, as an inserted `<option selected>` would.
+ * @param {HTMLSelectElement} select
+ * @param {boolean} preserve
+ */
+function apply_default_select_value(select, preserve) {
+	// @ts-expect-error
+	var value = select.__defaultValue;
+	var multiple = select.multiple;
+	var values = multiple ? value ?? [] : null;
+
+	if (multiple && !is_array(values)) return;
+
+	var index = select.selectedIndex;
+	var selected = preserve && multiple ? new Set(select.selectedOptions) : null;
 
 	for (var option of select.options) {
 		var option_value = get_option_value(option);
-		var is_selected = select.multiple
-			? /** @type {any[]} */ (values).includes(option_value)
-			: is(option_value, value);
-		set_selected(option, is_selected);
+		set_selected(
+			option,
+			multiple ? /** @type {any[]} */ (values).includes(option_value) : is(option_value, value)
+		);
 	}
 
+	if (!preserve) return;
+
 	if (selected !== null) {
-		for (option of select.options) option.selected = selected.has(option);
+		for (option of select.options) {
+			var was_selected = selected.has(option);
+			if (option.selected !== was_selected) option.selected = was_selected;
+		}
+	} else if (select.selectedIndex !== index) {
+		select.selectedIndex = index;
 	}
 }
 
@@ -91,11 +112,10 @@ export function select_option(select, value, mounting = false) {
 }
 
 /**
- * Selects the correct option(s) if `value` is given,
- * and then sets up a mutation observer to sync the
- * current selection to the dom when it changes. Such
- * changes could for example occur when options are
- * inside an `#each` block.
+ * Sets up a mutation observer to sync the current selection
+ * and default to the dom when the options change, for example
+ * when they are inside an `#each` block. Called once per `<select>`,
+ * by the compiled output or by `attribute_effect` for spreads.
  * @param {HTMLSelectElement} select
  */
 export function init_select(select) {
@@ -107,7 +127,7 @@ export function init_select(select) {
 		if (entries.every(is_selectedcontent_mutation)) return;
 
 		if ('__defaultValue' in select) {
-			set_default_select_value(select, select.__defaultValue, false);
+			apply_default_select_value(select, false);
 		}
 
 		if ('__value' in select) {
@@ -204,8 +224,6 @@ export function bind_select_value(select, get, set = get) {
 		select.__value = value;
 		mounting = false;
 	});
-
-	init_select(select);
 }
 
 /** @param {HTMLOptionElement} option */
diff --git a/packages/svelte/tests/runtime-runes/samples/form-default-value-select/_config.js b/packages/svelte/tests/runtime-runes/samples/form-default-value-select/_config.js
index 5d0c964969..784d20a0ce 100644
--- a/packages/svelte/tests/runtime-runes/samples/form-default-value-select/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/form-default-value-select/_config.js
@@ -62,7 +62,7 @@ export default test({
 		button('add').click();
 		flushSync();
 		await Promise.resolve();
-		check(selects[3], [true, false]);
+		check(selects[3], [false, true]);
 		assert.equal(selects[3].options[1].defaultSelected, true);
 
 		reset.click();
diff --git a/packages/svelte/tests/runtime-runes/samples/select-default-value-preserve-selection/_config.js b/packages/svelte/tests/runtime-runes/samples/select-default-value-preserve-selection/_config.js
new file mode 100644
index 0000000000..d409732ab8
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/select-default-value-preserve-selection/_config.js
@@ -0,0 +1,52 @@
+import { flushSync } from 'svelte';
+import { ok, test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [nothing, unmatched, spread, late, touched] = target.querySelectorAll('select');
+		const [change_default, change_spread, load, add] = target.querySelectorAll('button');
+		const p = target.querySelector('p');
+		ok(p);
+
+		assert.equal(nothing.selectedIndex, -1);
+		assert.equal(unmatched.selectedIndex, -1);
+		assert.equal(spread.selectedIndex, -1);
+		assert.equal(late.selectedIndex, -1);
+		assert.equal(touched.value, 'b');
+
+		change_default.click();
+		change_spread.click();
+		flushSync();
+
+		assert.equal(nothing.selectedIndex, -1);
+		assert.equal(unmatched.selectedIndex, -1);
+		assert.equal(spread.selectedIndex, -1);
+		assert.equal(spread.className, 'two');
+		assert.htmlEqual(p.innerHTML, 'zzz null');
+		assert.deepEqual(
+			[...nothing.options].map((option) => option.defaultSelected),
+			[true, false]
+		);
+
+		// a default change never moves the current selection
+		assert.equal(touched.value, 'b');
+
+		// a default whose option arrives later selects it
+		load.click();
+		flushSync();
+		await Promise.resolve();
+		assert.equal(late.value, 'b');
+
+		// a user selection survives option mutations and default changes
+		touched.options[2].selected = true;
+		touched.dispatchEvent(new Event('change', { bubbles: true }));
+		add.click();
+		flushSync();
+		await Promise.resolve();
+		assert.equal(touched.value, 'c');
+		assert.deepEqual(
+			[...touched.options].map((option) => option.defaultSelected),
+			[true, false, false, false]
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/select-default-value-preserve-selection/main.svelte b/packages/svelte/tests/runtime-runes/samples/select-default-value-preserve-selection/main.svelte
new file mode 100644
index 0000000000..c7fdea482c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/select-default-value-preserve-selection/main.svelte
@@ -0,0 +1,37 @@
+<script>
+	let unmatched = $state('zzz');
+	let nothing = $state(null);
+	let defaultValue = $state('b');
+	let props = $state({ defaultValue: 'b', class: 'one' });
+	let late = $state([]);
+	let options = $state(['a', 'b', 'c']);
+</script>
+
+<select value={null} {defaultValue}>
+	<option value="a">A</option>
+	<option value="b">B</option>
+</select>
+
+<select bind:value={unmatched} {defaultValue}>
+	<option value="a">A</option>
+	<option value="b">B</option>
+</select>
+
+<select {...props} bind:value={nothing}>
+	<option value="a">A</option>
+	<option value="b">B</option>
+</select>
+
+<select defaultValue="b">
+	{#each late as option}<option value={option}>{option}</option>{/each}
+</select>
+
+<select {defaultValue}>
+	{#each options as option}<option value={option}>{option}</option>{/each}
+</select>
+
+<button onclick={() => (defaultValue = 'a')}>change default</button>
+<button onclick={() => (props.class = 'two')}>change class</button>
+<button onclick={() => (late = ['a', 'b', 'c'])}>load options</button>
+<button onclick={() => options.push('d')}>add option</button>
+<p>{unmatched} {String(nothing)}</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/select-default-value-single-observer/_config.js b/packages/svelte/tests/runtime-runes/samples/select-default-value-single-observer/_config.js
new file mode 100644
index 0000000000..411a7868f7
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/select-default-value-single-observer/_config.js
@@ -0,0 +1,30 @@
+import { test } from '../../test';
+
+let observers = 0;
+const MutationObserver = globalThis.MutationObserver;
+
+export default test({
+	before_test() {
+		observers = 0;
+		globalThis.MutationObserver = class extends MutationObserver {
+			/** @param {MutationCallback} callback */
+			constructor(callback) {
+				super(callback);
+				observers++;
+			}
+		};
+	},
+
+	after_test() {
+		globalThis.MutationObserver = MutationObserver;
+	},
+
+	test({ assert, target }) {
+		const selects = target.querySelectorAll('select');
+		for (const select of selects) {
+			assert.equal(select.selectedIndex, 0);
+			assert.equal(select.options[1].defaultSelected, true);
+		}
+		assert.equal(observers, selects.length);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/select-default-value-single-observer/main.svelte b/packages/svelte/tests/runtime-runes/samples/select-default-value-single-observer/main.svelte
new file mode 100644
index 0000000000..7754b3c6fd
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/select-default-value-single-observer/main.svelte
@@ -0,0 +1,21 @@
+<script>
+	let value = $state('a');
+	let bound = $state('a');
+	let spread = $state('a');
+	let props = { defaultValue: 'b' };
+</script>
+
+<select {value} defaultValue="b">
+	<option value="a">A</option>
+	<option value="b">B</option>
+</select>
+
+<select bind:value={bound} defaultValue="b">
+	<option value="a">A</option>
+	<option value="b">B</option>
+</select>
+
+<select {...props} bind:value={spread}>
+	<option value="a">A</option>
+	<option value="b">B</option>
+</select>