fix: only update lazy properties that have actually changed (#10415)

* fix: only update lazy properties that have actually changed

* code golf

* changeset

* Add test

.changeset/rare-worms-hunt.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: only update lazy properties that have actually changed

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

@@ -786,10 +786,20 @@ export function get_first_element(block) {
  * @returns {void}
  */
 function update_each_item_block(block, item, index, type) {
+	const block_v = block.v;
 	if ((type & EACH_ITEM_REACTIVE) !== 0) {
-		set_signal_value(block.v, item);
+		set_signal_value(block_v, item);
-	} else if (is_lazy_property(block.v)) {
+	} else if (is_lazy_property(block_v)) {
-		block.v.o[block.v.p] = item;
+		// If we have lazy properties, it means that an array was used that has been
+		// proxied. Given this, we need to re-sync the old array by mutating the backing
+		// value to be the latest value to ensure the UI updates correctly. TODO: maybe
+		// we should bypass any internal mutation checks for this?
+		const o = block_v.o;
+		const p = block_v.p;
+		const prev = o[p];
+		if (prev !== item) {
+			o[p] = item;
+		}
 	}
 	const transitions = block.s;
 	const index_is_reactive = (type & EACH_INDEX_REACTIVE) !== 0;

packages/svelte/tests/runtime-runes/samples/readonly-state-push/Child.svelte

@@ -0,0 +1,7 @@
+<script>
+	let { array } = $props();
+</script>
+
+{#each array as number}
+	<p>{number}</p>
+{/each}

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

@@ -0,0 +1,14 @@
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [b1] = target.querySelectorAll('button');
+		b1.click();
+		await Promise.resolve();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			'<button>add</button><p>1</p><p>2</p><p>3</p><p>4</p><p>0</p>'
+		);
+	}
+});

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

@@ -0,0 +1,13 @@
+<script>
+  import Child from './Child.svelte';
+
+	let array = $state([1,2,3,4]);
+
+	const addNew = () => {
+		array.push(0)
+	}
+</script>
+
+<button onclick={addNew}>add</button>
+<Child {array} />
+