From 600a7ad48b6fd55b4bf79125611fcfe31ef9a83e Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Mon, 16 Jun 2025 17:03:53 +0200
Subject: [PATCH 1/2] fix: keep spread non-delegated event handlers up to date

#15961 introduced a regression where non-delegated events that were spread and updated were not getting updated. This fixes that by ensuring prev is actually updated to the most recent value
---
 .changeset/odd-readers-laugh.md               |  5 ++++
 .../client/dom/elements/attributes.js         |  8 +++---
 .../event-attribute-spread-update/_config.js  | 18 +++++++++++++
 .../event-attribute-spread-update/main.svelte | 27 +++++++++++++++++++
 4 files changed, 54 insertions(+), 4 deletions(-)
 create mode 100644 .changeset/odd-readers-laugh.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/event-attribute-spread-update/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/event-attribute-spread-update/main.svelte

diff --git a/.changeset/odd-readers-laugh.md b/.changeset/odd-readers-laugh.md
new file mode 100644
index 000000000000..e1401fdf1063
--- /dev/null
+++ b/.changeset/odd-readers-laugh.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: keep spread non-delegated event handlers up to date
diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index fcce0b444f49..b33b403e757a 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/packages/svelte/src/internal/client/dom/elements/attributes.js
@@ -483,8 +483,8 @@ export function attribute_effect(
 
 	block(() => {
 		var next = fn(...deriveds.map(get));
-
-		set_attributes(element, prev, next, css_hash, skip_warning);
+		/** @type {Record<string | symbol, any>} */
+		var current = set_attributes(element, prev, next, css_hash, skip_warning);
 
 		if (inited && is_select && 'value' in next) {
 			select_option(/** @type {HTMLSelectElement} */ (element), next.value, false);
@@ -495,7 +495,7 @@ export function attribute_effect(
 		}
 
 		for (let symbol of Object.getOwnPropertySymbols(next)) {
-			var n = next[symbol];
+			var n = (current[symbol] = next[symbol]);
 
 			if (symbol.description === ATTACHMENT_KEY && (!prev || n !== prev[symbol])) {
 				if (effects[symbol]) destroy_effect(effects[symbol]);
@@ -503,7 +503,7 @@ export function attribute_effect(
 			}
 		}
 
-		prev = next;
+		prev = current;
 	});
 
 	if (is_select) {
diff --git a/packages/svelte/tests/runtime-runes/samples/event-attribute-spread-update/_config.js b/packages/svelte/tests/runtime-runes/samples/event-attribute-spread-update/_config.js
new file mode 100644
index 000000000000..af03eed4c9e5
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/event-attribute-spread-update/_config.js
@@ -0,0 +1,18 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		const [change, increment] = target.querySelectorAll('button');
+
+		increment.click();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, '<button>change handlers</button><button>1 / 1</button>');
+
+		change.click();
+		flushSync();
+		increment.click();
+		flushSync();
+		assert.htmlEqual(target.innerHTML, '<button>change handlers</button><button>3 / 3</button>');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/event-attribute-spread-update/main.svelte b/packages/svelte/tests/runtime-runes/samples/event-attribute-spread-update/main.svelte
new file mode 100644
index 000000000000..32d4b242cc55
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/event-attribute-spread-update/main.svelte
@@ -0,0 +1,27 @@
+<script>
+	let delegated = $state(0);
+	let non_delegated = $state(0);
+	let attrs = $state({
+		onclick: () => {
+			delegated += 1;
+		},
+		onclickcapture: () => {
+			non_delegated += 1;
+		}
+	});
+</script>
+
+<button
+	onclick={() =>
+		(attrs = {
+			onclick: () => {
+				delegated += 2;
+			},
+			onclickcapture: () => {
+				non_delegated += 2;
+			}
+		})}
+>
+	change handlers
+</button>
+<button {...attrs}>{delegated} / {non_delegated}</button>

From ffded128e918ca43a3cf9d33746dba7b88b186d2 Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Tue, 17 Jun 2025 22:48:23 +0200
Subject: [PATCH 2/2] fix

---
 .../svelte/src/internal/client/dom/elements/attributes.js     | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/dom/elements/attributes.js b/packages/svelte/src/internal/client/dom/elements/attributes.js
index b33b403e757a..a663450b4a5c 100644
--- a/packages/svelte/src/internal/client/dom/elements/attributes.js
+++ b/packages/svelte/src/internal/client/dom/elements/attributes.js
@@ -495,12 +495,14 @@ export function attribute_effect(
 		}
 
 		for (let symbol of Object.getOwnPropertySymbols(next)) {
-			var n = (current[symbol] = next[symbol]);
+			var n = next[symbol];
 
 			if (symbol.description === ATTACHMENT_KEY && (!prev || n !== prev[symbol])) {
 				if (effects[symbol]) destroy_effect(effects[symbol]);
 				effects[symbol] = branch(() => attach(element, () => n));
 			}
+
+			current[symbol] = n;
 		}
 
 		prev = current;