From 47e37af9951591856a03dfcb734d063840910c38 Mon Sep 17 00:00:00 2001
From: thribhuvan003 <thribhuvan003@gmail.com>
Date: Fri, 10 Jul 2026 18:42:20 +0530
Subject: [PATCH] fix: ensure a batch exists when a boundary resolves
 synchronously

---
 .changeset/hip-bats-flash.md                               | 5 +++++
 packages/svelte/src/internal/client/dom/blocks/boundary.js | 7 +++++--
 .../samples/flush-sync-component-init/_config.js           | 6 ++++++
 .../samples/flush-sync-component-init/main.svelte          | 7 +++++++
 4 files changed, 23 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/hip-bats-flash.md
 create mode 100644 packages/svelte/tests/runtime-runes/samples/flush-sync-component-init/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/flush-sync-component-init/main.svelte

diff --git a/.changeset/hip-bats-flash.md b/.changeset/hip-bats-flash.md
new file mode 100644
index 0000000000..7a005dd370
--- /dev/null
+++ b/.changeset/hip-bats-flash.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure a batch exists when a boundary resolves synchronously
diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index 0c0903ff52..e3a4b5703f 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -235,7 +235,7 @@ export class Boundary {
 					this.#pending_effect = null;
 				});
 
-				this.#resolve(/** @type {Batch} */ (current_batch));
+				this.#resolve(Batch.ensure());
 			}
 		});
 	}
@@ -257,7 +257,10 @@ export class Boundary {
 				const pending = /** @type {(anchor: Node) => void} */ (this.#props.pending);
 				this.#pending_effect = branch(() => pending(this.#anchor));
 			} else {
-				this.#resolve(/** @type {Batch} */ (current_batch));
+				// `current_batch` can be null here — e.g. a `flushSync()` at the top
+				// level of a component script flushes and clears the batch while
+				// we are still rendering (#18522) — so ensure one exists
+				this.#resolve(Batch.ensure());
 			}
 		} catch (error) {
 			this.error(error);
diff --git a/packages/svelte/tests/runtime-runes/samples/flush-sync-component-init/_config.js b/packages/svelte/tests/runtime-runes/samples/flush-sync-component-init/_config.js
new file mode 100644
index 0000000000..0054b8993c
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/flush-sync-component-init/_config.js
@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+// https://github.com/sveltejs/svelte/issues/18522
+export default test({
+	html: `<h1>Hello!</h1>`
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/flush-sync-component-init/main.svelte b/packages/svelte/tests/runtime-runes/samples/flush-sync-component-init/main.svelte
new file mode 100644
index 0000000000..523d72ae69
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/flush-sync-component-init/main.svelte
@@ -0,0 +1,7 @@
+<script>
+	import { flushSync } from 'svelte';
+
+	flushSync();
+</script>
+
+<h1>Hello!</h1>