Merge 47e37af995 into 44a7813730

.changeset/hip-bats-flash.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure a batch exists when a boundary resolves synchronously

packages/svelte/src/internal/client/dom/blocks/boundary.js

@@ -286,7 +286,7 @@ export class Boundary {
 					this.#pending_effect = null;
 				});
 
-				this.#resolve(/** @type {Batch} */ (current_batch));
+				this.#resolve(Batch.ensure());
 			}
 		});
 	}
@@ -308,7 +308,10 @@ 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);

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>`
+});

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>