fix: flush when pending boundaries resolve

Author: Rich-Harris

.changeset/swift-starfishes-knock.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: flush when pending boundaries resolve

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

@@ -33,6 +33,7 @@ import { Batch, current_batch, effect_pending_updates } from '../../reactivity/b
 import { internal_set, source } from '../../reactivity/sources.js';
 import { tag } from '../../dev/tracing.js';
 import { createSubscriber } from '../../../../reactivity/create-subscriber.js';
+import { flushSync } from 'svelte';
 
 /**
  * @typedef {{
@@ -285,6 +286,10 @@ export class Boundary {
 				this.#anchor.before(this.#offscreen_fragment);
 				this.#offscreen_fragment = null;
 			}
+
+			queue_micro_task(() => {
+				Batch.ensure().flush();
+			});
 		}
 	}
 

packages/svelte/tests/runtime-runes/samples/async-attachment-in-block/_config.js

@@ -0,0 +1,11 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['client', 'hydrate'],
+
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<div>attachment ran</div>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-attachment-in-block/main.svelte

@@ -0,0 +1,9 @@
+{#if await true}
+	<div
+		{@attach (node) => {
+			node.textContent = 'attachment ran';
+		}}
+	>
+		attachment did not run
+	</div>
+{/if}