fix: set initial value for `open` binding in `<details>`

.changeset/strange-apricots-happen.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: set initial value for `open` binding in `<details>`

packages/svelte/src/compiler/phases/bindings.js

@@ -175,6 +175,7 @@ export const binding_properties = {
 	textContent: {},
 	open: {
 		event: 'toggle',
+		type: 'set',
 		valid_elements: ['details']
 	},
 	value: {

packages/svelte/tests/runtime-runes/samples/details-binding-initial/_config.js

@@ -0,0 +1,11 @@
+import { ok, test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		const details_el = target.querySelector('details');
+
+		ok(details_el);
+
+		assert.strictEqual(details_el.open, true);
+	}
+});

packages/svelte/tests/runtime-runes/samples/details-binding-initial/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	let open = $state(true);
+</script>
+
+<details bind:open>
+	 <summary>Details</summary>
+	 ...
+</details>