fix: better event delegation behavior around multiple listeners (#12024)

follow up to #12014, which wasn't quite the right fix. It would mean that delegated listeners between two manual listeners wouldn't be called at the correct time.

Author: dummdidumm

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

@@ -42,7 +42,7 @@ export function create_event(event_name, dom, handler, options) {
 	 * @this {EventTarget}
 	 */
 	function target_handler(/** @type {Event} */ event) {
-		if (!options.capture && /** @type {Event & {__root: any}} */ (event).__root === undefined) {
+		if (!options.capture) {
 			// Only call in the bubble phase, else delegated events would be called before the capturing events
 			handle_event_propagation(dom, event);
 		}
@@ -165,13 +165,15 @@ export function handle_event_propagation(handler_element, event) {
 		}
 
 		if (at_idx <= handler_idx) {
-			// +1 because at_idx is the element which was already handled, and there can only be one delegated event per element.
-			// Avoids on:click and onclick on the same event resulting in onclick being fired twice.
-			path_idx = at_idx + 1;
+			path_idx = at_idx;
 		}
 	}
 
 	current_target = /** @type {Element} */ (path[path_idx] || event.target);
+	// there can only be one delegated event per element, and we either already handled the current target,
+	// or this is the very first target in the chain which has a non-delegated listener, in which case it's safe
+	// to handle a possible delegated event on it later (through the root delegation listener for example).
+	if (current_target === handler_element) return;
 
 	// Proxy currentTarget to correct target
 	define_property(event, 'currentTarget', {
@@ -190,6 +192,7 @@ export function handle_event_propagation(handler_element, event) {
 		 * @type {unknown[]}
 		 */
 		var other_errors = [];
+
 		while (current_target !== null) {
 			/** @type {null | Element} */
 			var parent_element =
@@ -214,12 +217,7 @@ export function handle_event_propagation(handler_element, event) {
 					throw_error = error;
 				}
 			}
-			if (
-				event.cancelBubble ||
-				parent_element === handler_element ||
-				parent_element === null ||
-				current_target === handler_element
-			) {
+			if (event.cancelBubble || parent_element === handler_element || parent_element === null) {
 				break;
 			}
 			current_target = parent_element;

packages/svelte/tests/runtime-runes/samples/event-attribute-delegation-5/_config.js

@@ -7,8 +7,8 @@ export default test({
 		btn?.click();
 		await Promise.resolve();
 		assert.deepEqual(logs, [
-			'button onclick',
 			'button on:click',
+			'button onclick',
 			'inner div on:click',
 			'outer div onclick'
 		]);

packages/svelte/tests/runtime-runes/samples/event-on-2/_config.js

@@ -11,6 +11,6 @@ export default test({
 
 		b1?.dispatchEvent(keydown);
 		flushSync();
-		assert.deepEqual(logs, ['parent keydown']);
+		assert.deepEqual(logs, ['one', 'two', 'three', 'parent keydown', 'wrapper']);
 	}
 });

packages/svelte/tests/runtime-runes/samples/event-on-2/main.svelte

@@ -1,20 +1,23 @@
 <script>
-  import { on } from "svelte/events";
+	import { on } from 'svelte/events';
+	import Wrapper from './wrapper.svelte';
 
-  function handleParentKeyDown() {
-    console.log("parent keydown");
-  }
-  function keydownOne(node) {
-    on(node, "keydown", (e) => {});
-  }
-  function keydownTwo(node) {
-    on(node, "keydown", (e) => {});
-  }
-  function keydownThree(node) {
-    on(node, "keydown", (e) => {});
-  }
+	function handleParentKeyDown() {
+		console.log('parent keydown');
+	}
+	function keydownOne(node) {
+		on(node, 'keydown', (e) => console.log('one'));
+	}
+	function keydownTwo(node) {
+		on(node, 'keydown', (e) => console.log('two'));
+	}
+	function keydownThree(node) {
+		on(node, 'keydown', (e) => console.log('three'));
+	}
 </script>
 
-<div onkeydown={handleParentKeyDown}>
-  <button use:keydownOne use:keydownTwo use:keydownThree>button</button>
-</div>
+<Wrapper>
+	<div onkeydown={handleParentKeyDown}>
+		<button use:keydownOne use:keydownTwo use:keydownThree>button</button>
+	</div>
+</Wrapper>

packages/svelte/tests/runtime-runes/samples/event-on-2/wrapper.svelte

@@ -0,0 +1,3 @@
+<div on:keydown={() => console.log('wrapper')}>
+	<slot />
+</div>