From 5446ba81e4c4b47fe1859f0e05f9c09f04f65864 Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Tue, 6 Feb 2024 15:51:35 +0000
Subject: [PATCH 1/4] fix: only update lazy properties that have actually
 changed

---
 packages/svelte/src/internal/client/each.js | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/each.js b/packages/svelte/src/internal/client/each.js
index 032edad201b0..a2b3ee0f9683 100644
--- a/packages/svelte/src/internal/client/each.js
+++ b/packages/svelte/src/internal/client/each.js
@@ -789,7 +789,16 @@ function update_each_item_block(block, item, index, type) {
 	if ((type & EACH_ITEM_REACTIVE) !== 0) {
 		set_signal_value(block.v, item);
 	} 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;

From d7c2d502744f09dc207da272b0e9778e48993dc5 Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Tue, 6 Feb 2024 15:52:47 +0000
Subject: [PATCH 2/4] code golf

---
 packages/svelte/src/internal/client/each.js | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/packages/svelte/src/internal/client/each.js b/packages/svelte/src/internal/client/each.js
index a2b3ee0f9683..8485f519582e 100644
--- a/packages/svelte/src/internal/client/each.js
+++ b/packages/svelte/src/internal/client/each.js
@@ -786,15 +786,16 @@ 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);
-	} else if (is_lazy_property(block.v)) {
+		set_signal_value(block_v, item);
+	} else if (is_lazy_property(block_v)) {
 		// 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 o = block_v.o;
+		const p = block_v.p;
 		const prev = o[p];
 		if (prev !== item) {
 			o[p] = item;

From 06b308f8925ece2eada2b834f883c3758e968eb4 Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Tue, 6 Feb 2024 15:53:04 +0000
Subject: [PATCH 3/4] changeset

---
 .changeset/rare-worms-hunt.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/rare-worms-hunt.md

diff --git a/.changeset/rare-worms-hunt.md b/.changeset/rare-worms-hunt.md
new file mode 100644
index 000000000000..e0963bd6af21
--- /dev/null
+++ b/.changeset/rare-worms-hunt.md
@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: only update lazy properties that have actually changed

From c9d8bccffe1db2e86232ad7ced4cabb79b709407 Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Tue, 6 Feb 2024 16:15:22 +0000
Subject: [PATCH 4/4] Add test

---
 .../samples/readonly-state-push/Child.svelte       |  7 +++++++
 .../samples/readonly-state-push/_config.js         | 14 ++++++++++++++
 .../samples/readonly-state-push/main.svelte        | 13 +++++++++++++
 3 files changed, 34 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/readonly-state-push/Child.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/readonly-state-push/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/readonly-state-push/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/readonly-state-push/Child.svelte b/packages/svelte/tests/runtime-runes/samples/readonly-state-push/Child.svelte
new file mode 100644
index 000000000000..dc79fc19bf58
--- /dev/null
+++ b/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}
diff --git a/packages/svelte/tests/runtime-runes/samples/readonly-state-push/_config.js b/packages/svelte/tests/runtime-runes/samples/readonly-state-push/_config.js
new file mode 100644
index 000000000000..3d3895312722
--- /dev/null
+++ b/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>'
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/readonly-state-push/main.svelte b/packages/svelte/tests/runtime-runes/samples/readonly-state-push/main.svelte
new file mode 100644
index 000000000000..51c0940b9793
--- /dev/null
+++ b/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} />
+