simpler isCustomElement & skip extra function call

- pass options.customElement down to mount_component
- remove expensive isCustomElement check
- only call add_render_callback if not customElement

src/compiler/compile/render_dom/index.ts

@@ -485,7 +485,7 @@ export default function dom(
 
 					${css.code && b`this.shadowRoot.innerHTML = \`<style>${css.code.replace(/\\/g, '\\\\')}${options.dev ? `\n/*# sourceMappingURL=${css.map.toUrl()} */` : ''}</style>\`;`}
 
-					@init(this, { target: this.shadowRoot, props: ${init_props} }, ${definition}, ${has_create_fragment ? 'create_fragment' : 'null'}, ${not_equal}, ${prop_indexes}, ${dirty});
+					@init(this, { target: this.shadowRoot, props: ${init_props}, customElement: true }, ${definition}, ${has_create_fragment ? 'create_fragment' : 'null'}, ${not_equal}, ${prop_indexes}, ${dirty});
 
 					${dev_props_check}
 

src/runtime/internal/Component.ts

@@ -53,16 +53,14 @@ export function claim_component(block, parent_nodes) {
 	block && block.l(parent_nodes);
 }
 
-export function mount_component(component, target, anchor) {
+export function mount_component(component, target, anchor, customElement) {
 	const { fragment, on_mount, on_destroy, after_update } = component.$$;
-	const isCustomElement = Object.getPrototypeOf(component.constructor) === SvelteElement;
 
 	fragment && fragment.m(target, anchor);
 
+	if (!customElement) {
 		// onMount happens before the initial afterUpdate
 		add_render_callback(() => {
-		// custom element on_mount is called in connectedCallback
-		if (isCustomElement) return;
 
 			const new_on_destroy = on_mount.map(run).filter(is_function);
 			if (on_destroy) {
@@ -74,6 +72,7 @@ export function mount_component(component, target, anchor) {
 			}
 			component.$$.on_mount = [];
 		});
+	}
 
 	after_update.forEach(add_render_callback);
 }
@@ -161,7 +160,7 @@ export function init(component, options, instance, create_fragment, not_equal, p
 		}
 
 		if (options.intro) transition_in(component.$$.fragment);
-		mount_component(component, options.target, options.anchor);
+		mount_component(component, options.target, options.anchor, options.customElement);
 		flush();
 	}
 

test/js/samples/css-shadow-dom-keyframes/expected.js

@@ -40,7 +40,8 @@ class Component extends SvelteElement {
 			this,
 			{
 				target: this.shadowRoot,
-				props: attribute_to_object(this.attributes)
+				props: attribute_to_object(this.attributes),
+				customElement: true
 			},
 			null,
 			create_fragment,