fix: widen ownership upon property access if necessary

In case of something like `foo = bar.map(...)`, foo would have ownership
of the array itself, while the individual items would have ownership
of the component that created the proxy. That means if we later do
`foo[0].baz = 42`, we could get a false-positive ownership violation,
since the two proxies are not connected to each other via the parent
relationship. For this reason, we need to widen the ownership of the
children upon access when we detect they are not connected.

Fixes #13137

.changeset/five-birds-check.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: widen ownership upon property access if necessary

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

@@ -131,6 +131,22 @@ export function proxy(value, parent = null, prev) {
 
 			if (s !== undefined) {
 				var v = get(s);
+
+				// In case of something like `foo = bar.map(...)`, foo would have ownership
+				// of the array itself, while the individual items would have ownership
+				// of the component that created the proxy. That means if we later do
+				// `foo[0].baz = 42`, we could get a false-positive ownership violation,
+				// since the two proxies are not connected to each other via the parent
+				// relationship. For this reason, we need to widen the ownership of the
+				// children upon access when we detect they are not connected.
+				if (DEV) {
+					/** @type {ProxyMetadata | undefined} */
+					var prop_metadata = v?.[STATE_SYMBOL_METADATA];
+					if (prop_metadata && prop_metadata?.parent !== metadata) {
+						widen_ownership(metadata, prop_metadata);
+					}
+				}
+
 				return v === UNINITIALIZED ? undefined : v;
 			}
 

packages/svelte/tests/runtime-runes/samples/non-local-mutation-with-binding-7/Component1.svelte

@@ -0,0 +1,13 @@
+<script>
+	import Component2 from './Component2.svelte';
+
+	let { rows = $bindable([]) } = $props();
+
+	let rows2 = $state([]);
+
+	$effect(() => {
+		rows2 = rows.slice();
+	});
+</script>
+
+<Component2 bind:rows={rows2} />

packages/svelte/tests/runtime-runes/samples/non-local-mutation-with-binding-7/Component2.svelte

@@ -0,0 +1,7 @@
+<script>
+	let { rows = $bindable([]) } = $props();
+</script>
+
+{#if rows.length}
+	<input type="checkbox" bind:checked={rows[0].check} />
+{/if}

packages/svelte/tests/runtime-runes/samples/non-local-mutation-with-binding-7/_config.js

@@ -0,0 +1,22 @@
+import { flushSync } from 'svelte';
+import { ok, test } from '../../test';
+
+// Tests that proxies widen ownership correctly even if not directly connected to each other
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	test({ assert, target, warnings }) {
+		const input = target.querySelector('input');
+		ok(input);
+
+		input.checked = true;
+		input.dispatchEvent(new Event('input', { bubbles: true }));
+		flushSync();
+
+		assert.deepEqual(warnings, []);
+	},
+
+	warnings: []
+});

packages/svelte/tests/runtime-runes/samples/non-local-mutation-with-binding-7/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	import Component1 from './Component1.svelte';
+
+	let rows = $state([{}]);
+</script>
+
+<Component1 bind:rows />