fix: account for mounting when select_option in attribute_effect (#16309)

Author: paoloricciuti

.changeset/curvy-carrots-prove.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: account for mounting when `select_option` in `attribute_effect`

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

@@ -514,9 +514,10 @@ export function attribute_effect(
 
 	if (is_select) {
 		var select = /** @type {HTMLSelectElement} */ (element);
-
+		var mounting = true;
 		effect(() => {
-			select_option(select, /** @type {Record<string | symbol, any>} */ (prev).value);
+			select_option(select, /** @type {Record<string | symbol, any>} */ (prev).value, mounting);
+			mounting = false;
 			init_select(select);
 		});
 	}

packages/svelte/tests/runtime-runes/samples/select-spread-option-selected/_config.js

@@ -0,0 +1,9 @@
+import { ok, test } from '../../test';
+
+export default test({
+	async test({ assert, target, instance }) {
+		const select = target.querySelector('select');
+		ok(select);
+		assert.equal(select.selectedIndex, 1);
+	}
+});

packages/svelte/tests/runtime-runes/samples/select-spread-option-selected/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	let others = {onclick: ()=> {}}
+</script>
+
+<select {...others}>
+	<option>o1</option>
+	<option selected>o2</option>
+</select>