fix: replay load and error events on load during hydration (#11642)

* fix: replay load and error events on load during hydration

* oops

* fix replacement logic

* make less evasive

* address feedback

* address feedback

* address feedback

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

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

* address feedback

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

Co-authored-by: Rich Harris <rich.harris@vercel.com>

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

Co-authored-by: Rich Harris <rich.harris@vercel.com>

* address more feedback

* address more feedback

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/wise-kids-wash.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: replay load and error events on load during hydration

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

@@ -8,7 +8,12 @@ import {
 } from '../../../../utils/ast.js';
 import { binding_properties } from '../../../bindings.js';
 import { clean_nodes, determine_namespace_for_children, infer_namespace } from '../../utils.js';
-import { DOMProperties, PassiveEvents, VoidElements } from '../../../constants.js';
+import {
+	DOMProperties,
+	LoadErrorElements,
+	PassiveEvents,
+	VoidElements
+} from '../../../constants.js';
 import { is_custom_element_node, is_element_node } from '../../../nodes.js';
 import * as b from '../../../../utils/builders.js';
 import {
@@ -1904,6 +1909,7 @@ export const template_visitors = {
 		let is_content_editable = false;
 		let has_content_editable_binding = false;
 		let img_might_be_lazy = false;
+		let might_need_event_replaying = false;
 
 		if (is_custom_element) {
 			// cloneNode is faster, but it does not instantiate the underlying class of the
@@ -1936,6 +1942,9 @@ export const template_visitors = {
 				attributes.push(attribute);
 				needs_input_reset = true;
 				needs_content_reset = true;
+				if (LoadErrorElements.includes(node.name)) {
+					might_need_event_replaying = true;
+				}
 			} else if (attribute.type === 'ClassDirective') {
 				class_directives.push(attribute);
 			} else if (attribute.type === 'StyleDirective') {
@@ -1958,6 +1967,8 @@ export const template_visitors = {
 					) {
 						has_content_editable_binding = true;
 					}
+				} else if (attribute.type === 'UseDirective' && LoadErrorElements.includes(node.name)) {
+					might_need_event_replaying = true;
 				}
 				context.visit(attribute);
 			}
@@ -2010,6 +2021,12 @@ export const template_visitors = {
 		} else {
 			for (const attribute of /** @type {import('#compiler').Attribute[]} */ (attributes)) {
 				if (is_event_attribute(attribute)) {
+					if (
+						(attribute.name === 'onload' || attribute.name === 'onerror') &&
+						LoadErrorElements.includes(node.name)
+					) {
+						might_need_event_replaying = true;
+					}
 					serialize_event_attribute(attribute, context);
 					continue;
 				}
@@ -2058,6 +2075,10 @@ export const template_visitors = {
 		serialize_class_directives(class_directives, node_id, context, is_attributes_reactive);
 		serialize_style_directives(style_directives, node_id, context, is_attributes_reactive);
 
+		if (might_need_event_replaying) {
+			context.state.after_update.push(b.stmt(b.call('$.replay_events', node_id)));
+		}
+
 		context.state.template.push('>');
 
 		/** @type {import('../types.js').SourceLocation[]} */

packages/svelte/src/compiler/phases/3-transform/server/transform-server.js

@@ -11,6 +11,7 @@ import * as b from '../../../utils/builders.js';
 import is_reference from 'is-reference';
 import {
 	ContentEditableBindings,
+	LoadErrorElements,
 	VoidElements,
 	WhitespaceInsensitiveAttributes
 } from '../../constants.js';
@@ -1845,6 +1846,7 @@ function serialize_element_attributes(node, context) {
 	// Use the index to keep the attributes order which is important for spreading
 	let class_attribute_idx = -1;
 	let style_attribute_idx = -1;
+	let events_to_capture = new Set();
 
 	for (const attribute of node.attributes) {
 		if (attribute.type === 'Attribute') {
@@ -1861,8 +1863,15 @@ function serialize_element_attributes(node, context) {
 				}
 				content = { escape: true, expression: serialize_attribute_value(attribute.value, context) };
 
-				// omit event handlers
+				// omit event handlers except for special cases
-			} else if (!is_event_attribute(attribute)) {
+			} else if (is_event_attribute(attribute)) {
+				if (
+					(attribute.name === 'onload' || attribute.name === 'onerror') &&
+					LoadErrorElements.includes(node.name)
+				) {
+					events_to_capture.add(attribute.name);
+				}
+			} else {
 				if (attribute.name === 'class') {
 					class_attribute_idx = attributes.length;
 				} else if (attribute.name === 'style') {
@@ -1960,6 +1969,15 @@ function serialize_element_attributes(node, context) {
 		} else if (attribute.type === 'SpreadAttribute') {
 			attributes.push(attribute);
 			has_spread = true;
+			if (LoadErrorElements.includes(node.name)) {
+				events_to_capture.add('onload');
+				events_to_capture.add('onerror');
+			}
+		} else if (attribute.type === 'UseDirective') {
+			if (LoadErrorElements.includes(node.name)) {
+				events_to_capture.add('onload');
+				events_to_capture.add('onerror');
+			}
 		} else if (attribute.type === 'ClassDirective') {
 			class_directives.push(attribute);
 		} else if (attribute.type === 'StyleDirective') {
@@ -2042,6 +2060,12 @@ function serialize_element_attributes(node, context) {
 		}
 	}
 
+	if (events_to_capture.size !== 0) {
+		for (const event of events_to_capture) {
+			context.state.template.push(t_string(` ${event}="this.__e=event"`));
+		}
+	}
+
 	return content;
 }
 

packages/svelte/src/compiler/phases/constants.js

@@ -61,6 +61,18 @@ export const WhitespaceInsensitiveAttributes = ['class', 'style'];
 
 export const ContentEditableBindings = ['textContent', 'innerHTML', 'innerText'];
 
+export const LoadErrorElements = [
+	'body',
+	'embed',
+	'iframe',
+	'img',
+	'link',
+	'object',
+	'script',
+	'style',
+	'track'
+];
+
 export const SVGElements = [
 	'altGlyph',
 	'altGlyphDef',

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

@@ -1,6 +1,34 @@
 import { render_effect } from '../../reactivity/effects.js';
 import { all_registered_events, root_event_handles } from '../../render.js';
 import { define_property, is_array } from '../../utils.js';
+import { hydrating } from '../hydration.js';
+
+/**
+ * SSR adds onload and onerror attributes to catch those events before the hydration.
+ * This function detects those cases, removes the attributes and replays the events.
+ * @param {HTMLElement} dom
+ */
+export function replay_events(dom) {
+	if (!hydrating) return;
+
+	if (dom.onload) {
+		dom.removeAttribute('onload');
+	}
+	if (dom.onerror) {
+		dom.removeAttribute('onerror');
+	}
+	// @ts-expect-error
+	const event = dom.__e;
+	if (event !== undefined) {
+		// @ts-expect-error
+		dom.__e = undefined;
+		queueMicrotask(() => {
+			if (dom.isConnected) {
+				dom.dispatchEvent(event);
+			}
+		});
+	}
+}
 
 /**
  * @param {string} event_name

packages/svelte/src/internal/client/dom/operations.js

@@ -71,6 +71,8 @@ export function init_operations() {
 	element_prototype.__className = '';
 	// @ts-expect-error
 	element_prototype.__attributes = null;
+	// @ts-expect-error
+	element_prototype.__e = undefined;
 
 	if (DEV) {
 		// @ts-expect-error

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

@@ -30,7 +30,7 @@ export {
 	handle_lazy_img
 } from './dom/elements/attributes.js';
 export { set_class, set_svg_class, set_mathml_class, toggle_class } from './dom/elements/class.js';
-export { event, delegate } from './dom/elements/events.js';
+export { event, delegate, replay_events } from './dom/elements/events.js';
 export { autofocus, remove_textarea_child } from './dom/elements/misc.js';
 export { set_style } from './dom/elements/style.js';
 export { animation, transition } from './dom/elements/transitions.js';

packages/svelte/tests/html_equal.js

@@ -80,6 +80,8 @@ export function normalize_html(
 			.replace(/(<!(--)?.*?\2>)/g, preserveComments ? '$1' : '')
 			.replace(/(data-svelte-h="[^"]+")/g, removeDataSvelte ? '' : '$1')
 			.replace(/>[ \t\n\r\f]+</g, '><')
+			// Strip out the special onload/onerror hydration events from the test output
+			.replace(/\s?onerror="this.__e=event"|\s?onload="this.__e=event"/g, '')
 			.trim();
 		clean_children(node);
 		return node.innerHTML.replace(/<\/?noscript\/?>/g, '');