fix: ensure spread events are always added (#11535)

In edge cases it may happen that set_attributes is re-run before the effect is executed. In that case the render effect which initiates this re-run will destroy the inner effect and it will never run. But because next and prev may have the same keys, the event would not get added again and it would get lost. We prevent this by using a root effect.
The added test case doesn't fail for some reason without this fix, but it does fail when you test it out manually, so I still added it.
Found through #10359 (comment)

Author: dummdidumm

.changeset/two-brooms-fail.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: ensure spread events are added even when rerunning spread immediately

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

@@ -4,8 +4,7 @@ import { get_descriptors, get_prototype_of, map_get, map_set } from '../../utils
 import { AttributeAliases, DelegatedEvents, namespace_svg } from '../../../../constants.js';
 import { delegate } from './events.js';
 import { autofocus } from './misc.js';
-import { effect } from '../../reactivity/effects.js';
-import { run } from '../../../shared/utils.js';
+import { effect, effect_root } from '../../reactivity/effects.js';
 import * as w from '../../warnings.js';
 
 /**
@@ -112,7 +111,7 @@ export function set_attributes(element, prev, next, lowercase_attributes, css_ha
 
 	// @ts-expect-error
 	var attributes = /** @type {Record<string, unknown>} **/ (element.__attributes ??= {});
-	/** @type {Array<() => void>} */
+	/** @type {Array<[string, any, () => void]>} */
 	var events = [];
 
 	for (key in next) {
@@ -145,7 +144,7 @@ export function set_attributes(element, prev, next, lowercase_attributes, css_ha
 			if (value != null) {
 				if (!delegated) {
 					if (!prev) {
-						events.push(() => element.addEventListener(event_name, value, opts));
+						events.push([key, value, () => element.addEventListener(event_name, value, opts)]);
 					} else {
 						element.addEventListener(event_name, value, opts);
 					}
@@ -193,7 +192,22 @@ export function set_attributes(element, prev, next, lowercase_attributes, css_ha
 	// On the first run, ensure that events are added after bindings so
 	// that their listeners fire after the binding listeners
 	if (!prev) {
-		effect(() => events.forEach(run));
+		// In edge cases it may happen that set_attributes is re-run before the
+		// effect is executed. In that case the render effect which initiates this
+		// re-run will destroy the inner effect and it will never run. But because
+		// next and prev may have the same keys, the event would not get added again
+		// and it would get lost. We prevent this by using a root effect.
+		const destroy_root = effect_root(() => {
+			effect(() => {
+				if (!element.isConnected) return;
+				for (const [key, value, evt] of events) {
+					if (next[key] === value) {
+						evt();
+					}
+				}
+				destroy_root();
+			});
+		});
 	}
 
 	return next;

packages/svelte/tests/runtime-runes/samples/event-spread-rerun/_config.js

@@ -0,0 +1,15 @@
+import { test, ok } from '../../test';
+
+export default test({
+	mode: ['client'],
+
+	async test({ assert, logs, target }) {
+		const input = target.querySelector('input');
+		ok(input);
+
+		input.value = 'foo';
+		await input.dispatchEvent(new Event('input'));
+
+		assert.deepEqual(logs, ['hi']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/event-spread-rerun/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	let rest = $state(undefined);
+</script>
+
+<input {...rest} oninput={() => console.log('hi')}>
+<!-- after input and inside template so that attribute spread reruns immediately -->
+{!rest ? (rest = {}) : false}