fix: process batches created in effect and immediately flushed (#18194)

no changeset because the bug this fixes hasn't been released

packages/svelte/src/internal/client/reactivity/batch.js

@@ -372,12 +372,11 @@ export class Batch {
 		// once more in that case - most of the time this will just clean up dirty branches.
 		if (this.#roots.length > 0) {
 			const batch = (next_batch ??= this);
+			batches.add(batch);
 			batch.#roots.push(...this.#roots.filter((r) => !batch.#roots.includes(r)));
 		}
 
 		if (next_batch !== null) {
-			batches.add(next_batch);
-
 			if (DEV) {
 				for (const source of this.current.keys()) {
 					/** @type {Set<Source>} */ (source_stacks).add(source);
@@ -728,20 +727,14 @@ export class Batch {
 	static ensure() {
 		if (current_batch === null) {
 			const batch = (current_batch = new Batch());
+			batches.add(batch);
 
-			if (!is_processing) {
-				batches.add(current_batch);
-
-				if (!is_flushing_sync) {
-					queue_micro_task(() => {
-						if (batch.#started) {
-							// a flushSync happened in the meantime
-							return;
-						}
-
+			if (!is_processing && !is_flushing_sync) {
+				queue_micro_task(() => {
+					if (!batch.#started) {
 						batch.flush();
-					});
-				}
+					}
+				});
 			}
 		}
 

packages/svelte/tests/runtime-runes/samples/async-flushsync-in-effect/_config.js

@@ -0,0 +1,25 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		await tick();
+
+		const [increment, shift] = target.querySelectorAll('button');
+
+		increment.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, '<button>clicks: 0</button><button>shift</button> 0');
+
+		shift.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, '<button>clicks: 1</button><button>shift</button> 1');
+
+		shift.click();
+		await tick();
+
+		assert.htmlEqual(target.innerHTML, '<button>clicks: 2</button><button>shift</button> 2');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-flushsync-in-effect/main.svelte

@@ -0,0 +1,25 @@
+<script lang="ts">
+	import { flushSync } from 'svelte';
+
+	let count = $state(0);
+
+	const queue: Array<() => void> = [];
+
+	$effect(() => {
+		if (count === 1) {
+			count = 2;
+			flushSync();
+		}
+	})
+
+	function push(v: number) {
+		if (v === 0) return v;
+		return new Promise(r => queue.push(() => r(v)));
+	}
+</script>
+
+<button onclick={() => count += 1}>
+	clicks: {count}
+</button>
+<button onclick={() => queue.shift()?.()}>shift</button>
+{await push(count)}