fix: catch delegated events from elements moved outside the container (#10060)

fixes #9777

Author: dummdidumm

.changeset/dirty-bats-punch.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle delegated events of elements moved outside the container

packages/svelte/src/internal/client/render.js

@@ -1280,6 +1280,10 @@ function handle_event_propagation(root_element, event) {
 	const handled_at = event.__root;
 	if (handled_at) {
 		const at_idx = path.indexOf(handled_at);
+		if (at_idx !== -1 && root_element === document) {
+			// This is the fallback document listener but the event was already handled -> ignore
+			return;
+		}
 		if (at_idx < path.indexOf(root_element)) {
 			path_idx = at_idx;
 		}
@@ -2778,13 +2782,17 @@ export function mount(component, options) {
 		set_current_hydration_fragment(previous_hydration_fragment);
 	}
 	const bound_event_listener = handle_event_propagation.bind(null, container);
+	const bound_document_event_listener = handle_event_propagation.bind(null, document);
 
 	/** @param {Array<string>} events */
 	const event_handle = (events) => {
 		for (let i = 0; i < events.length; i++) {
 			const event_name = events[i];
 			if (!registered_events.has(event_name)) {
 				registered_events.add(event_name);
+				// Add the event listener to both the container and the document.
+				// The container listener ensures we catch events from within in case
+				// the outer content stops propagation of the event.
 				container.addEventListener(
 					event_name,
 					bound_event_listener,
@@ -2794,6 +2802,17 @@ export function mount(component, options) {
 						  }
 						: undefined
 				);
+				// The document listener ensures we catch events that originate from elements that were
+				// manually moved outside of the container (e.g. via manual portals).
+				document.addEventListener(
+					event_name,
+					bound_document_event_listener,
+					PassiveDelegatedEvents.includes(event_name)
+						? {
+								passive: true
+						  }
+						: undefined
+				);
 			}
 		}
 	};

packages/svelte/tests/runtime-runes/samples/event-listener-moved-outside-container/_config.js

@@ -0,0 +1,23 @@
+import { test } from '../../test';
+
+// Tests that event delegation still works when the element with the event listener is moved outside the container
+export default test({
+	async test({ assert, target }) {
+		const btn1 = target.parentElement?.querySelector('button');
+		const btn2 = target.querySelector('button');
+
+		btn1?.click();
+		await Promise.resolve();
+		assert.htmlEqual(
+			target.parentElement?.innerHTML ?? '',
+			'<main><div><button>clicks: 1</button></div></main><button>clicks: 1</button>'
+		);
+
+		btn2?.click();
+		await Promise.resolve();
+		assert.htmlEqual(
+			target.parentElement?.innerHTML ?? '',
+			'<main><div><button>clicks: 2</button></div></main><button>clicks: 2</button>'
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/event-listener-moved-outside-container/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	let count = $state(0);
+	let el;
+	$effect(() => {
+		document.getElementsByTagName('body')[0].appendChild(el);
+	})
+</script>
+
+<div>
+	<button bind:this={el} onclick={() => count++}>clicks: {count}</button>
+	<button onclick={() => count++}>clicks: {count}</button>
+</div>