fix: remove spreaded event handlers when they become nullish (#14546)

Fixes #14539

Author: paoloricciuti

.changeset/itchy-cooks-yawn.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: remove spreaded event handlers when they become nullish

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -281,6 +281,9 @@ export function set_attributes(
 					element[`__${event_name}`] = value;
 					delegate([event_name]);
 				}
+			} else if (delegated) {
+				// @ts-ignore
+				element[`__${event_name}`] = undefined;
 			}
 		} else if (key === 'style' && value != null) {
 			element.style.cssText = value + '';

packages/svelte/tests/runtime-runes/samples/remove-spreaded-handlers/_config.js

@@ -0,0 +1,32 @@
+import { ok, test } from '../../test';
+import { flushSync } from 'svelte';
+
+export default test({
+	async test({ assert, target, instance }) {
+		const p = target.querySelector('p');
+		const btn = target.querySelector('button');
+		const input = target.querySelector('input');
+		ok(p);
+
+		flushSync(() => {
+			btn?.click();
+		});
+		assert.equal(p.innerHTML, '1');
+
+		flushSync(() => {
+			input?.click();
+		});
+		flushSync(() => {
+			btn?.click();
+		});
+		assert.equal(p.innerHTML, '1');
+
+		flushSync(() => {
+			input?.click();
+		});
+		flushSync(() => {
+			btn?.click();
+		});
+		assert.equal(p.innerHTML, '2');
+	}
+});

packages/svelte/tests/runtime-runes/samples/remove-spreaded-handlers/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	let checked = $state(true)
+	let count = $state(0)
+
+	const onclick = $derived(checked ? (() => { count++ }) : undefined)
+</script>
+
+<p>
+	{count}
+</p>
+<input type="checkbox" bind:checked />
+<button {...{onclick}}></button>