fix

packages/svelte/src/internal/client/dom/blocks/svelte-component.js

@@ -1,7 +1,10 @@
 /** @import { TemplateNode, Dom, Effect } from '#client' */
 import { EFFECT_TRANSPARENT } from '../../constants.js';
 import { block, branch, pause_effect } from '../../reactivity/effects.js';
+import { active_effect } from '../../runtime.js';
 import { hydrate_next, hydrate_node, hydrating } from '../hydration.js';
+import { should_defer_append } from '../operations.js';
+import { add_boundary_callback, find_boundary } from './boundary.js';
 
 /**
  * @template P
@@ -24,16 +27,47 @@ export function component(node, get_component, render_fn) {
 	/** @type {Effect | null} */
 	var effect;
 
-	block(() => {
-		if (component === (component = get_component())) return;
+	/** @type {DocumentFragment | null} */
+	var offscreen_fragment = null;
+
+	/** @type {Effect | null} */
+	var pending_effect = null;
 
+	var boundary = find_boundary(active_effect);
+
+	function commit() {
 		if (effect) {
 			pause_effect(effect);
 			effect = null;
 		}
 
+		if (offscreen_fragment) {
+			anchor.before(offscreen_fragment);
+			offscreen_fragment = null;
+		}
+
+		effect = pending_effect;
+	}
+
+	block(() => {
+		if (component === (component = get_component())) return;
+
 		if (component) {
-			effect = branch(() => render_fn(anchor, component));
+			var defer = boundary !== null && should_defer_append();
+			var target = anchor;
+
+			if (defer) {
+				offscreen_fragment = document.createDocumentFragment();
+				offscreen_fragment.append((target = document.createComment('')));
+			}
+
+			pending_effect = branch(() => render_fn(anchor, component));
+
+			if (defer) {
+				add_boundary_callback(boundary, commit);
+			} else {
+				commit();
+			}
 		}
 	}, EFFECT_TRANSPARENT);