fix: correctly reflect readonly proxy marker (#9893)

Author: trueadm

.changeset/rotten-bags-type.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly reflect readonly proxy marker

packages/svelte/src/internal/client/proxy/proxy.js

@@ -149,8 +149,9 @@ const handler = {
 	},
 
 	get(target, prop, receiver) {
-		if (DEV && prop === READONLY_SYMBOL) return target[READONLY_SYMBOL];
-
+		if (DEV && prop === READONLY_SYMBOL) {
+			return Reflect.get(target, READONLY_SYMBOL);
+		}
 		const metadata = target[STATE_SYMBOL];
 		let s = metadata.s.get(prop);
 
@@ -184,6 +185,9 @@ const handler = {
 	},
 
 	has(target, prop) {
+		if (DEV && prop === READONLY_SYMBOL) {
+			return Reflect.has(target, READONLY_SYMBOL);
+		}
 		if (prop === STATE_SYMBOL) {
 			return true;
 		}

packages/svelte/tests/runtime-runes/samples/state-readonly/Component.svelte

@@ -0,0 +1,11 @@
+<script>
+	import Component2 from './Component2.svelte';
+	const {state} = $props();
+
+	function render(state) {
+    return state
+  }
+</script>
+
+<Component2 state={render(state)} />
+

packages/svelte/tests/runtime-runes/samples/state-readonly/Component2.svelte

@@ -0,0 +1,5 @@
+<script>
+	const {state} = $props();
+</script>
+
+{state}

packages/svelte/tests/runtime-runes/samples/state-readonly/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	async test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		await btn?.click();
+		assert.htmlEqual(target.innerHTML, `<button></button>\n[object Object]`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/state-readonly/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	import Component from './Component.svelte';
+
+	let state = $state();
+</script>
+
+<button onclick={() => {
+	state = {}
+}}></button>
+{#if state}
+	<Component state={state} />
+{/if}