From 7e6a3bfea8f7023d725288c9ef82cb67d810ffe0 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 9 Sep 2025 22:05:25 -0400
Subject: [PATCH 1/5] fix: flush effects scheduled during boundary's pending
 phase

---
 .changeset/ninety-pandas-move.md               |  5 +++++
 .../src/internal/client/dom/blocks/boundary.js | 10 +++++++++-
 .../samples/async-attachment/Inner.svelte      | 10 ++++++++++
 .../samples/async-attachment/_config.js        | 18 ++++++++++++++++++
 .../samples/async-attachment/main.svelte       | 16 ++++++++++++++++
 5 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 .changeset/ninety-pandas-move.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-attachment/Inner.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-attachment/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-attachment/main.svelte

diff --git a/.changeset/ninety-pandas-move.md b/.changeset/ninety-pandas-move.md
new file mode 100644
index 000000000000..65f57ddbbf85
--- /dev/null
+++ b/.changeset/ninety-pandas-move.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: flush effects scheduled during boundary's pending phase
diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index b7f180378270..659f6014b804 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -23,7 +23,7 @@ import { queue_micro_task } from '../task.js';
 import * as e from '../../errors.js';
 import * as w from '../../warnings.js';
 import { DEV } from 'esm-env';
-import { Batch, effect_pending_updates } from '../../reactivity/batch.js';
+import { Batch, current_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';
@@ -259,6 +259,14 @@ export class Boundary {
 				this.#anchor.before(this.#offscreen_fragment);
 				this.#offscreen_fragment = null;
 			}
+
+			const batch = current_batch;
+
+			if (batch) {
+				queue_micro_task(() => {
+					batch.flush();
+				});
+			}
 		}
 	}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/async-attachment/Inner.svelte b/packages/svelte/tests/runtime-runes/samples/async-attachment/Inner.svelte
new file mode 100644
index 000000000000..b9b9d7a3d08b
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-attachment/Inner.svelte
@@ -0,0 +1,10 @@
+<script>
+	function renderContent(node) {
+		node.textContent = 'foo';
+	}
+ 
+	const test = await Promise.resolve('foo');
+</script>
+
+<p>{test}</p>
+<div {@attach renderContent}></div>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-attachment/_config.js b/packages/svelte/tests/runtime-runes/samples/async-attachment/_config.js
new file mode 100644
index 000000000000..f6b48b38b14a
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-attachment/_config.js
@@ -0,0 +1,18 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<button>toggle</button> <p>foo</p><div>foo</div>');
+
+		const [toggle] = target.querySelectorAll('button');
+		toggle.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<button>toggle</button>');
+
+		toggle.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<button>toggle</button> <p>foo</p><div>foo</div>');
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-attachment/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-attachment/main.svelte
new file mode 100644
index 000000000000..6cef6e8f5c8a
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-attachment/main.svelte
@@ -0,0 +1,16 @@
+<script>
+	import Inner from './Inner.svelte';
+
+	let show = $state(true);
+</script>
+
+<svelte:boundary>
+	<button onclick={() => show = !show}>toggle</button>
+	{#if show}
+		<Inner />
+	{/if}
+
+	{#snippet pending()}
+		<p>pending</p>
+	{/snippet}
+</svelte:boundary>

From bcd3acad210c22b8c39d6408e94207ee1754cf21 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 9 Sep 2025 22:22:22 -0400
Subject: [PATCH 2/5] simpler fix

---
 .../svelte/src/internal/client/dom/blocks/boundary.js     | 8 --------
 packages/svelte/src/internal/client/reactivity/batch.js   | 2 +-
 2 files changed, 1 insertion(+), 9 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index 659f6014b804..28f35b0b6662 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -259,14 +259,6 @@ export class Boundary {
 				this.#anchor.before(this.#offscreen_fragment);
 				this.#offscreen_fragment = null;
 			}
-
-			const batch = current_batch;
-
-			if (batch) {
-				queue_micro_task(() => {
-					batch.flush();
-				});
-			}
 		}
 	}
 
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 5176a4f74b0d..555286e53ab3 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -685,7 +685,7 @@ export function suspend() {
 			batch.activate();
 			batch.decrement();
 		} else {
-			batch.deactivate();
+			batch.flush();
 		}
 
 		unset_context();

From e36137d161e88013f116e5b9e2b141d7d86ca1a7 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Wed, 10 Sep 2025 08:18:54 -0400
Subject: [PATCH 3/5] add async-effect-after-await test

---
 .../samples/async-effect-after-await/Child.svelte        | 7 +++++++
 .../samples/async-effect-after-await/_config.js          | 9 +++++++++
 .../samples/async-effect-after-await/main.svelte         | 9 +++++++++
 3 files changed, 25 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-effect-after-await/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
new file mode 100644
index 000000000000..682f7a063179
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
@@ -0,0 +1,7 @@
+<script>
+	await 1;
+
+	$effect(() => {
+		console.log('hello');
+	});
+</script>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
new file mode 100644
index 000000000000..81548a25ea67
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
@@ -0,0 +1,9 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, logs }) {
+		await tick();
+		assert.deepEqual(logs, ['hello']);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/main.svelte
new file mode 100644
index 000000000000..d4b67f880380
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/main.svelte
@@ -0,0 +1,9 @@
+<script>
+	import Child from './Child.svelte';
+</script>
+
+<svelte:boundary>
+	<Child />
+
+	{#snippet pending()}{/snippet}
+</svelte:boundary>

From f41dad72226104a417149442323df39126925dfd Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Wed, 10 Sep 2025 12:33:08 -0400
Subject: [PATCH 4/5] failing test

---
 .../samples/async-effect-after-await/Child.svelte           | 6 +++++-
 .../samples/async-effect-after-await/_config.js             | 3 ++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
index 682f7a063179..758ebc00040c 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/Child.svelte
@@ -1,7 +1,11 @@
 <script>
+	$effect(() => {
+		console.log('before');
+	});
+
 	await 1;
 
 	$effect(() => {
-		console.log('hello');
+		console.log('after');
 	});
 </script>
diff --git a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
index 81548a25ea67..0908b6a9feb9 100644
--- a/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/async-effect-after-await/_config.js
@@ -3,7 +3,8 @@ import { test } from '../../test';
 
 export default test({
 	async test({ assert, logs }) {
+		assert.deepEqual(logs, []);
 		await tick();
-		assert.deepEqual(logs, ['hello']);
+		assert.deepEqual(logs, ['before', 'after']);
 	}
 });

From d50b4d955a8633aceeddad5711f65434e02c3a39 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Mon, 22 Sep 2025 14:58:02 -0400
Subject: [PATCH 5/5] add another test

---
 .../async-boundary-coordination/_config.js    | 46 +++++++++++++++++++
 .../async-boundary-coordination/main.svelte   | 35 ++++++++++++++
 2 files changed, 81 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js
new file mode 100644
index 000000000000..f3e60e7b3267
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js
@@ -0,0 +1,46 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<p>hello from server</p>
+				<p>hello from server</p>
+				<p>hello from server</p>
+				<p>hello from server</p>
+			`
+		);
+
+		const [button1, button2] = target.querySelectorAll('button');
+
+		button1.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<p>hello from browser</p>
+				<p>hello from browser</p>
+				<p>hello from server</p>
+				<p>hello from server</p>
+			`
+		);
+
+		button2.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<p>hello from browser</p>
+				<p>hello from browser</p>
+				<p>hello from browser</p>
+				<p>hello from browser</p>
+			`
+		);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte
new file mode 100644
index 000000000000..3986c450466e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte
@@ -0,0 +1,35 @@
+<script>
+	let browser = typeof window !== 'undefined';
+
+	let a = Promise.withResolvers();
+	let b = Promise.withResolvers();
+
+	if (!browser) {
+		a.resolve(true);
+		b.resolve(true);
+	}
+</script>
+
+<button onclick={() => a.resolve(true)}>
+	resolve a
+</button>
+
+<button onclick={() => b.resolve(true)}>
+	resolve b
+</button>
+
+<svelte:boundary>
+	{#if await a.promise}
+		<p>hello from {browser ? 'browser' : 'server'}</p>
+	{/if}
+
+	<p>hello from {browser ? 'browser' : 'server'}</p>
+</svelte:boundary>
+
+<svelte:boundary>
+	{#if await b.promise}
+		<p>hello from {browser ? 'browser' : 'server'}</p>
+	{/if}
+
+	<p>hello from {browser ? 'browser' : 'server'}</p>
+</svelte:boundary>