fix: ensure proxied array length is updated (#9782)

* fix: ensure proxied array length is updated

* fix: ensure proxied array length is updated

* add test

* lint

.changeset/cool-ants-leave.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure proxied array length is updated

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

@@ -139,23 +139,34 @@ const handler = {
 			target[READONLY_SYMBOL] = value;
 			return true;
 		}
-
 		const metadata = target[STATE_SYMBOL];
-
 		const s = metadata.s.get(prop);
 		if (s !== undefined) set(s, proxy(value));
+		const is_array = metadata.a;
+		const not_has = !(prop in target);
 
-		if (metadata.a && prop === 'length') {
+		if (is_array && prop === 'length') {
 			for (let i = value; i < target.length; i += 1) {
 				const s = metadata.s.get(i + '');
 				if (s !== undefined) set(s, UNINITIALIZED);
 			}
 		}
-
-		if (!(prop in target)) increment(metadata.v);
+		if (not_has) {
+			increment(metadata.v);
+		}
 		// @ts-ignore
 		target[prop] = value;
 
+		// If we have mutated an array directly, we might need to
+		// signal that length has also changed too.
+		if (is_array && not_has) {
+			const ls = metadata.s.get('length');
+			const length = target.length;
+			if (ls !== undefined && ls.v !== length) {
+				set(ls, length);
+			}
+		}
+
 		return true;
 	},
 

packages/svelte/tests/runtime-runes/samples/proxy-array-length/_config.js

@@ -0,0 +1,9 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_if_ssr: 'permanent',
+	html: `
+		<input><input><input><div>3</div>
+	`
+});

packages/svelte/tests/runtime-runes/samples/proxy-array-length/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	let values = $state(['foo', 'bar', 'baz']);
+	let elements = $state([]);
+	let nums = $state([1,2,3]);
+</script>
+
+{#each values as value, i}
+	<input bind:this={elements[i]} bind:value={values[i]} />
+{/each}
+<div>{elements.length}</div>