From 41e34bed360bceb8419160a4cc9962f830834643 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sun, 5 Oct 2025 17:04:07 -0400
Subject: [PATCH 1/3] fix: flush when pending boundaries resolve

---
 .changeset/swift-starfishes-knock.md                  |  5 +++++
 .../svelte/src/internal/client/dom/blocks/boundary.js |  5 +++++
 .../samples/async-attachment-in-block/_config.js      | 11 +++++++++++
 .../samples/async-attachment-in-block/main.svelte     |  9 +++++++++
 4 files changed, 30 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 000000000000..790b3fe697aa
--- /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 6dde40d88959..efd88b600b08 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/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();
+			});
 		}
 	}
 
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 000000000000..97da1bf83516
--- /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 000000000000..027b980bd181
--- /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}

From 2f22c615dd0f1b996b2ed74bb3ea34af9da58f8b Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sun, 5 Oct 2025 17:06:31 -0400
Subject: [PATCH 2/3] note to self

---
 packages/svelte/src/internal/client/dom/blocks/boundary.js | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index efd88b600b08..ed0c89c1e953 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -287,6 +287,9 @@ export class Boundary {
 				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();
 			});

From dfe4f4371812f0e3396c4902f9f46024e3fcc441 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sun, 5 Oct 2025 17:23:57 -0400
Subject: [PATCH 3/3] Update
 packages/svelte/src/internal/client/dom/blocks/boundary.js

---
 packages/svelte/src/internal/client/dom/blocks/boundary.js | 1 -
 1 file changed, 1 deletion(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index ed0c89c1e953..88f4a85e20dc 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -33,7 +33,6 @@ 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 {{