From 2b49a5e3c0851ddba87d78b48551bd5f40e1ee13 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sun, 5 Oct 2025 17:59:58 -0400
Subject: [PATCH] fix: flush when pending boundaries resolve (#16897)

* fix: flush when pending boundaries resolve

* note to self

* Update packages/svelte/src/internal/client/dom/blocks/boundary.js
---
 .changeset/swift-starfishes-knock.md                  |  5 +++++
 .../svelte/src/internal/client/dom/blocks/boundary.js |  7 +++++++
 .../samples/async-attachment-in-block/_config.js      | 11 +++++++++++
 .../samples/async-attachment-in-block/main.svelte     |  9 +++++++++
 4 files changed, 32 insertions(+)
 create mode 100644 .changeset/swift-starfishes-knock.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-attachment-in-block/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-attachment-in-block/main.svelte

diff --git a/.changeset/swift-starfishes-knock.md b/.changeset/swift-starfishes-knock.md
new file mode 100644
index 0000000000..790b3fe697
--- /dev/null
+++ b/.changeset/swift-starfishes-knock.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: flush when pending boundaries resolve
diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index 6dde40d889..88f4a85e20 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -285,6 +285,13 @@ export class Boundary {
 				this.#anchor.before(this.#offscreen_fragment);
 				this.#offscreen_fragment = null;
 			}
+
+			// TODO this feels like a little bit of a kludge, but until we
+			// overhaul the boundary/batch relationship it's probably
+			// the most pragmatic solution available to us
+			queue_micro_task(() => {
+				Batch.ensure().flush();
+			});
 		}
 	}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/async-attachment-in-block/_config.js b/packages/svelte/tests/runtime-runes/samples/async-attachment-in-block/_config.js
new file mode 100644
index 0000000000..97da1bf835
--- /dev/null
+++ b/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>');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-attachment-in-block/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-attachment-in-block/main.svelte
new file mode 100644
index 0000000000..027b980bd1
--- /dev/null
+++ b/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}