fix: improve ff handling of lazy images

.changeset/plenty-zoos-fix.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: improve handling of lazy image elements

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

@@ -1888,30 +1888,22 @@ export const template_visitors = {
 		let needs_special_value_handling = node.name === 'option' || node.name === 'select';
 		let is_content_editable = false;
 		let has_content_editable_binding = false;
+		let img_might_be_lazy = false;
 
-		if (
+		if (is_custom_element) {
 			// cloneNode is faster, but it does not instantiate the underlying class of the
 			// custom element until the template is connected to the dom, which would
 			// cause problems when setting properties on the custom element.
 			// Therefore we need to use importNode instead, which doesn't have this caveat.
-			is_custom_element ||
-			// If we have an <img loading="lazy"> occurance, we need to use importNode for FF
-			// otherwise, the image won't be lazy. If we detect an attribute for "loading" then
-			// just fallback to using importNode. Also if we have a spread attribute on the img,
-			// then it might contain this property, so we also need to fallback there too.
-			(node.name === 'img' &&
-				node.attributes.some(
-					(attribute) =>
-						attribute.type === 'SpreadAttribute' ||
-						(attribute.type === 'Attribute' && attribute.name === 'loading')
-				))
-		) {
 			metadata.context.template_needs_import_node = true;
 		}
 
 		for (const attribute of node.attributes) {
 			if (attribute.type === 'Attribute') {
 				attributes.push(attribute);
+				if (node.name === 'img' && attribute.name === 'loading') {
+					img_might_be_lazy = true;
+				}
 				if (
 					(attribute.name === 'value' || attribute.name === 'checked') &&
 					!is_text_attribute(attribute)
@@ -1988,6 +1980,9 @@ export const template_visitors = {
 		// Then do attributes
 		let is_attributes_reactive = false;
 		if (node.metadata.has_spread) {
+			if (node.name === 'img') {
+				img_might_be_lazy = true;
+			}
 			serialize_element_spread_attributes(
 				attributes,
 				context,
@@ -2039,6 +2034,11 @@ export const template_visitors = {
 			}
 		}
 
+		// Apply the src and loading attributes for <img> elements after the element is appended to the document
+		if (img_might_be_lazy) {
+			context.state.after_update.push(b.stmt(b.call('$.handle_lazy_img', node_id)));
+		}
+
 		// class/style directives must be applied last since they could override class/style attributes
 		serialize_class_directives(class_directives, node_id, context, is_attributes_reactive);
 		serialize_style_directives(style_directives, node_id, context, is_attributes_reactive);

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

@@ -333,3 +333,23 @@ function srcset_url_equal(element, srcset) {
 		)
 	);
 }
+
+/**
+ * @param {HTMLImageElement} element
+ * @returns {void}
+ */
+export function handle_lazy_img(element) {
+	// If we're using an image that has a lazy loading attribute, we need to apply
+	// the loading and src after the img element has been appended to the document.
+	// Otherwise the lazy behaviour will not work due to our cloneNode heuristic for
+	// templates.
+	if (!hydrating && element.loading === 'lazy') {
+		var src = element.src;
+		element.removeAttribute('loading');
+		element.removeAttribute('src');
+		requestAnimationFrame(() => {
+			element.loading = 'lazy';
+			element.src = src;
+		});
+	}
+}

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

@@ -40,7 +40,7 @@ export function hydrate_anchor(node) {
 	var current = /** @type {Node | null} */ (node);
 
 	// TODO this could have false positives, if a user comment consisted of `[`. need to tighten that up
-	if (/** @type {Comment} */ (current)?.data !== HYDRATION_START) {
+	if (/** @type {Comment} */ (current).data !== HYDRATION_START) {
 		return node;
 	}
 

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

@@ -26,7 +26,8 @@ export {
 	set_attributes,
 	set_custom_element_data,
 	set_dynamic_element_attributes,
-	set_xlink_attribute
+	set_xlink_attribute,
+	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';