fix: coordinate mount of snippets with await expressions (#17021)

* fix: coordinate mount of snippets with await expressions

* try this

* deduplicate

---------

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

.changeset/huge-poets-tickle.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: coordinate mount of snippets with await expressions

packages/svelte/src/internal/client/dom/blocks/boundary.js

@@ -38,6 +38,7 @@ import { Batch, effect_pending_updates } from '../../reactivity/batch.js';
 import { internal_set, source } from '../../reactivity/sources.js';
 import { tag } from '../../dev/tracing.js';
 import { createSubscriber } from '../../../../reactivity/create-subscriber.js';
+import { create_text } from '../operations.js';
 
 /**
  * @typedef {{
@@ -92,6 +93,9 @@ export class Boundary {
 	/** @type {DocumentFragment | null} */
 	#offscreen_fragment = null;
 
+	/** @type {TemplateNode | null} */
+	#pending_anchor = null;
+
 	#local_pending_count = 0;
 	#pending_count = 0;
 
@@ -155,8 +159,10 @@ export class Boundary {
 					this.#hydrate_resolved_content();
 				}
 			} else {
+				var anchor = this.#get_anchor();
+
 				try {
-					this.#main_effect = branch(() => children(this.#anchor));
+					this.#main_effect = branch(() => children(anchor));
 				} catch (error) {
 					this.error(error);
 				}
@@ -167,6 +173,10 @@ export class Boundary {
 					this.#pending = false;
 				}
 			}
+
+			return () => {
+				this.#pending_anchor?.remove();
+			};
 		}, flags);
 
 		if (hydrating) {
@@ -194,9 +204,11 @@ export class Boundary {
 		this.#pending_effect = branch(() => pending(this.#anchor));
 
 		Batch.enqueue(() => {
+			var anchor = this.#get_anchor();
+
 			this.#main_effect = this.#run(() => {
 				Batch.ensure();
-				return branch(() => this.#children(this.#anchor));
+				return branch(() => this.#children(anchor));
 			});
 
 			if (this.#pending_count > 0) {
@@ -211,6 +223,19 @@ export class Boundary {
 		});
 	}
 
+	#get_anchor() {
+		var anchor = this.#anchor;
+
+		if (this.#pending) {
+			this.#pending_anchor = create_text();
+			this.#anchor.before(this.#pending_anchor);
+
+			anchor = this.#pending_anchor;
+		}
+
+		return anchor;
+	}
+
 	/**
 	 * Returns `true` if the effect exists inside a boundary whose pending snippet is shown
 	 * @returns {boolean}
@@ -252,6 +277,7 @@ export class Boundary {
 
 		if (this.#main_effect !== null) {
 			this.#offscreen_fragment = document.createDocumentFragment();
+			this.#offscreen_fragment.append(/** @type {TemplateNode} */ (this.#pending_anchor));
 			move_effect(this.#main_effect, this.#offscreen_fragment);
 		}
 

packages/svelte/tests/runtime-runes/samples/async-snippet-coordinated-mount/Child.svelte

@@ -0,0 +1,7 @@
+<script>
+	let { children, push } = $props();
+	let message = await push('hello from child');
+</script>
+
+<p>message: {message}</p>
+{@render children()}

packages/svelte/tests/runtime-runes/samples/async-snippet-coordinated-mount/_config.js

@@ -0,0 +1,25 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [shift] = target.querySelectorAll('button');
+
+		shift.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, `<button>shift</button><p>loading...</p>`);
+
+		shift.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>shift</button>
+				<p>message: hello from child</p>
+				<p>hello from parent</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-snippet-coordinated-mount/main.svelte

@@ -0,0 +1,21 @@
+<script>
+	import Child from './Child.svelte';
+	const resolvers = [];
+	function push(value) {
+		const { promise, resolve } = Promise.withResolvers();
+		resolvers.push(() => resolve(value));
+		return promise;
+	}
+</script>
+
+<button onclick={() => resolvers.shift()?.()}>shift</button>
+
+<svelte:boundary>
+	<Child {push}>
+		<p>{await push('hello from parent')}</p>
+	</Child>
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>