From 7c9ff8fc697a0723129d781e5bf116cf38e016f6 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 12 Mar 2026 09:28:43 -0400
Subject: [PATCH] fix: resolve boundary in correct batch when hydrating
 (#17914)

Fixes #17907. When hydrating, we were resolving the boundary in the
hydration batch rather than the batch created inside the
`queue_micro_task` inside `#hydrate_pending_content`. This meant that
effects got scheduled inside a batch that was already resolved.

### Before submitting the PR, please make sure you do the following

- [x] It's really useful if your PR references an issue where it is
discussed ahead of time. In many cases, features are absent for a
reason. For large changes, please create an RFC:
https://github.com/sveltejs/rfcs
- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### Tests and linting

- [x] Run the tests with `pnpm test` and lint the project with `pnpm
lint`
---
 .changeset/stale-loops-love.md                        |  5 +++++
 .../svelte/src/internal/client/dom/blocks/boundary.js |  8 ++------
 .../samples/effect-in-pending-boundary/Child.svelte   |  5 +++++
 .../samples/effect-in-pending-boundary/_config.js     |  7 +++++++
 .../samples/effect-in-pending-boundary/main.svelte    | 11 +++++++++++
 5 files changed, 30 insertions(+), 6 deletions(-)
 create mode 100644 .changeset/stale-loops-love.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/Child.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/main.svelte

diff --git a/.changeset/stale-loops-love.md b/.changeset/stale-loops-love.md
new file mode 100644
index 0000000000..d36392cca3
--- /dev/null
+++ b/.changeset/stale-loops-love.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: resolve boundary in correct batch when hydrating
diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index b38a3131ca..8046f1e222 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -218,8 +218,6 @@ export class Boundary {
 		this.is_pending = true;
 		this.#pending_effect = branch(() => pending(this.#anchor));
 
-		var batch = /** @type {Batch} */ (current_batch);
-
 		queue_micro_task(() => {
 			var fragment = (this.#offscreen_fragment = document.createDocumentFragment());
 			var anchor = create_text();
@@ -238,14 +236,12 @@ export class Boundary {
 					this.#pending_effect = null;
 				});
 
-				this.#resolve(batch);
+				this.#resolve(/** @type {Batch} */ (current_batch));
 			}
 		});
 	}
 
 	#render() {
-		var batch = /** @type {Batch} */ (current_batch);
-
 		try {
 			this.is_pending = this.has_pending_snippet();
 			this.#pending_count = 0;
@@ -262,7 +258,7 @@ export class Boundary {
 				const pending = /** @type {(anchor: Node) => void} */ (this.#props.pending);
 				this.#pending_effect = branch(() => pending(this.#anchor));
 			} else {
-				this.#resolve(batch);
+				this.#resolve(/** @type {Batch} */ (current_batch));
 			}
 		} catch (error) {
 			this.error(error);
diff --git a/packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/Child.svelte b/packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/Child.svelte
new file mode 100644
index 0000000000..0f18e43e56
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/Child.svelte
@@ -0,0 +1,5 @@
+<script lang="ts">
+	$effect(() => {
+		console.log('hello from child');
+	});
+</script>
diff --git a/packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/_config.js b/packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/_config.js
new file mode 100644
index 0000000000..21575231ee
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/_config.js
@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, logs }) {
+		assert.deepEqual(logs, ['hello from child']);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/main.svelte b/packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/main.svelte
new file mode 100644
index 0000000000..c4c0ef23ab
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/effect-in-pending-boundary/main.svelte
@@ -0,0 +1,11 @@
+<script>
+	import Child from './Child.svelte';
+</script>
+
+<svelte:boundary>
+	<Child />
+
+	{#snippet pending()}
+		<p>Loading...</p>
+	{/snippet}
+</svelte:boundary>