fix: simplify event delegation logic

Only delegate event attributes, and don't take into account bindings/actions while determining that. Never delegate `on:` directives. This simplifies the logic and makes it easier to explain, while avoiding any accidental breaking changes when upgrading from Svelte 4 to 5 without changing code.
Fixes #10165
Related to #9714

Author: dummdidumm

.changeset/rich-cobras-exist.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: simplify event delegation logic, only delegate event attributes

packages/svelte/src/compiler/phases/1-parse/state/element.js

@@ -128,8 +128,7 @@ export default function tag(parser) {
 					fragment: create_fragment(true),
 					metadata: {
 						svg: false,
-						has_spread: false,
-						can_delegate_events: null
+						has_spread: false
 					},
 					parent: null
 				}

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -59,35 +59,31 @@ function get_component_name(filename) {
 }
 
 /**
- * @param {Pick<import('#compiler').OnDirective, 'expression'| 'name' | 'modifiers'> & { type: string }} node
+ * Checks if given event attribute can be delegated/hoisted and returns the corresponding info if so
+ * @param {string} event_name
+ * @param {import('estree').Expression | null} handler
  * @param {import('./types').Context} context
  * @returns {null | import('#compiler').DelegatedEvent}
  */
-function get_delegated_event(node, context) {
-	const handler = node.expression;
-	const event_name = node.name;
-
+function get_delegated_event(event_name, handler, context) {
 	// Handle delegated event handlers. Bail-out if not a delegated event.
-	if (!handler || node.modifiers.includes('capture') || !DelegatedEvents.includes(event_name)) {
+	if (!handler || !DelegatedEvents.includes(event_name)) {
 		return null;
 	}
+
 	// If we are not working with a RegularElement, then bail-out.
 	const element = context.path.at(-1);
 	if (element?.type !== 'RegularElement') {
 		return null;
 	}
-	// If element says we can't delegate because we have multiple OnDirectives of the same type, bail-out.
-	if (!element.metadata.can_delegate_events) {
-		return null;
-	}
 
 	/** @type {import('#compiler').DelegatedEvent} */
 	const non_hoistable = { type: 'non-hoistable' };
 	/** @type {import('estree').FunctionExpression | import('estree').FunctionDeclaration | import('estree').ArrowFunctionExpression | null} */
 	let target_function = null;
 	let binding = null;
 
-	if (node.type === 'Attribute' && element.metadata.has_spread) {
+	if (element.metadata.has_spread) {
 		// event attribute becomes part of the dynamic spread array
 		return non_hoistable;
 	}
@@ -123,8 +119,7 @@ function get_delegated_event(node, context) {
 				if (element && event_name) {
 					if (
 						element.type !== 'RegularElement' ||
-						!determine_element_spread_and_delegatable(element).metadata.can_delegate_events ||
-						(element.metadata.has_spread && node.type === 'Attribute') ||
+						determine_element_spread(element).metadata.has_spread ||
 						!DelegatedEvents.includes(event_name)
 					) {
 						return non_hoistable;
@@ -183,7 +178,8 @@ function get_delegated_event(node, context) {
 		) {
 			return non_hoistable;
 		}
-		// If we referebnce the index within an each block, then bail-out.
+
+		// If we reference the index within an each block, then bail-out.
 		if (binding !== null && binding.initial?.type === 'EachBlock') {
 			return non_hoistable;
 		}
@@ -204,6 +200,7 @@ function get_delegated_event(node, context) {
 		}
 		visited_references.add(reference);
 	}
+
 	return { type: 'hoistable', function: target_function };
 }
 
@@ -858,21 +855,9 @@ const common_visitors = {
 		});
 
 		if (is_event_attribute(node)) {
-			/** @type {string[]} */
-			const modifiers = [];
 			const expression = node.value[0].expression;
 
-			let name = node.name.slice(2);
-
-			if (is_capture_event(name)) {
-				name = name.slice(0, -7);
-				modifiers.push('capture');
-			}
-
-			const delegated_event = get_delegated_event(
-				{ type: node.type, name, expression, modifiers },
-				context
-			);
+			const delegated_event = get_delegated_event(node.name.slice(2), expression, context);
 
 			if (delegated_event !== null) {
 				if (delegated_event.type === 'hoistable') {
@@ -1032,18 +1017,6 @@ const common_visitors = {
 			)
 		};
 	},
-	OnDirective(node, context) {
-		node.metadata = { delegated: null };
-		context.next();
-		const delegated_event = get_delegated_event(node, context);
-
-		if (delegated_event !== null) {
-			if (delegated_event.type === 'hoistable') {
-				delegated_event.function.metadata.hoistable = true;
-			}
-			node.metadata.delegated = delegated_event;
-		}
-	},
 	ArrowFunctionExpression: function_visitor,
 	FunctionExpression: function_visitor,
 	FunctionDeclaration: function_visitor,
@@ -1052,7 +1025,7 @@ const common_visitors = {
 			node.metadata.svg = true;
 		}
 
-		determine_element_spread_and_delegatable(node);
+		determine_element_spread(node);
 
 		// Special case: Move the children of <textarea> into a value attribute if they are dynamic
 		if (
@@ -1110,51 +1083,15 @@ const common_visitors = {
 };
 
 /**
- * Check if events on this element can theoretically be delegated. They can if there's no
- * possibility of an OnDirective and an event attribute on the same element, and if there's
- * no OnDirectives of the same type (the latter is a bit too strict because `on:click on:click on:keyup`
- * means that `on:keyup` can be delegated but we gloss over this edge case).
  * @param {import('#compiler').RegularElement} node
  */
-function determine_element_spread_and_delegatable(node) {
-	if (typeof node.metadata.can_delegate_events === 'boolean') {
-		return node; // did this already
-	}
-
-	let events = new Map();
+function determine_element_spread(node) {
 	let has_spread = false;
-	let has_on = false;
-	let has_action_or_bind = false;
 	for (const attribute of node.attributes) {
-		if (
-			attribute.type === 'OnDirective' ||
-			(attribute.type === 'Attribute' && is_event_attribute(attribute))
-		) {
-			let event_name = attribute.name;
-			if (attribute.type === 'Attribute') {
-				event_name = get_attribute_event_name(event_name);
-			}
-			events.set(event_name, (events.get(event_name) || 0) + 1);
-			if (!has_on && attribute.type === 'OnDirective') {
-				has_on = true;
-			}
-		} else if (!has_spread && attribute.type === 'SpreadAttribute') {
+		if (!has_spread && attribute.type === 'SpreadAttribute') {
 			has_spread = true;
-		} else if (
-			!has_action_or_bind &&
-			((attribute.type === 'BindDirective' && attribute.name !== 'this') ||
-				attribute.type === 'UseDirective')
-		) {
-			has_action_or_bind = true;
 		}
 	}
-	node.metadata.can_delegate_events =
-		// Actions/bindings need the old on:-events to fire in order
-		!has_action_or_bind &&
-		// spreading events means we don't know if there's an event attribute with the same name as an on:-event
-		!(has_spread && has_on) &&
-		// multiple on:-events/event attributes with the same name
-		![...events.values()].some((count) => count > 1);
 	node.metadata.has_spread = has_spread;
 
 	return node;

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -1309,7 +1309,7 @@ function serialize_event_handler(node, { state, visit }) {
 
 /**
  * Serializes an event handler function of the `on:` directive or an attribute starting with `on`
- * @param {Pick<import('#compiler').OnDirective, 'name' | 'modifiers' | 'expression' | 'metadata'>} node
+ * @param {{name: string; modifiers: string[]; expression: import('estree').Expression | null; delegated?: import('#compiler').DelegatedEvent | null; }} node
  * @param {import('../types.js').ComponentContext} context
  */
 function serialize_event(node, context) {
@@ -1318,9 +1318,9 @@ function serialize_event(node, context) {
 	if (node.expression) {
 		let handler = serialize_event_handler(node, context);
 		const event_name = node.name;
-		const delegated = node.metadata.delegated;
+		const delegated = node.delegated;
 
-		if (delegated !== null) {
+		if (delegated != null) {
 			let delegated_assignment;
 
 			if (!state.events.has(event_name)) {
@@ -1415,7 +1415,7 @@ function serialize_event_attribute(node, context) {
 			name: event_name,
 			expression: node.value[0].expression,
 			modifiers,
-			metadata: node.metadata
+			delegated: node.metadata.delegated
 		},
 		context
 	);

packages/svelte/src/compiler/types/template.d.ts

@@ -202,9 +202,6 @@ export interface OnDirective extends BaseNode {
 	/** The 'y' in `on:x={y}` */
 	expression: null | Expression;
 	modifiers: string[]; // TODO specify
-	metadata: {
-		delegated: null | DelegatedEvent;
-	};
 }
 
 export type DelegatedEvent =
@@ -288,12 +285,6 @@ export interface RegularElement extends BaseElement {
 		svg: boolean;
 		/** `true` if contains a SpreadAttribute */
 		has_spread: boolean;
-		/**
-		 * `true` if events on this element can theoretically be delegated. This doesn't necessarily mean that
-		 * a specific event will be delegated, as there are other factors which affect the final outcome.
-		 * `null` only until it was determined whether this element can be delegated or not.
-		 */
-		can_delegate_events: boolean | null;
 	};
 }
 

packages/svelte/tests/runtime-runes/samples/inspect-trace/_config.js

@@ -31,9 +31,7 @@ export default test({
 		b2.click();
 		await Promise.resolve();
 
-		assert.ok(
-			log[0].stack.startsWith('Error:') && log[0].stack.includes('HTMLButtonElement.on_click')
-		);
+		assert.ok(log[0].stack.startsWith('Error:') && log[0].stack.includes('HTMLButtonElement.'));
 		assert.deepEqual(log[1], 1);
 	}
 });