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

Author: dummdidumm

.changeset/odd-readers-laugh.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: keep spread non-delegated event handlers up to date

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);
@@ -495,15 +495,15 @@ 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]);
 				effects[symbol] = branch(() => attach(element, () => n));
 			}
 		}
 
-		prev = next;
+		prev = current;
 	});
 
 	if (is_select) {

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>');
+	}
+});

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>