fix: avoid other batches running with queued root effects of main batch

.changeset/sixty-glasses-try.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid other batches running with queued root effects of main batch

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

@@ -166,6 +166,8 @@ export class Batch {
 
 		for (const root of root_effects) {
 			this.#traverse_effect_tree(root, target);
+			// Note: #traverse_effect_tree runs block effects eagerly, which can schedule effects,
+			// which means queued_root_effects now may be filled again.
 		}
 
 		if (!this.is_fork) {
@@ -392,6 +394,10 @@ export class Batch {
 				// Re-run async/block effects that depend on distinct values changed in both batches
 				const others = [...batch.current.keys()].filter((s) => !this.current.has(s));
 				if (others.length > 0) {
+					// Avoid running queued root effects on the wrong branch
+					var prev_queued_root_effects = queued_root_effects;
+					queued_root_effects = [];
+
 					/** @type {Set<Value>} */
 					const marked = new Set();
 					/** @type {Map<Reaction, boolean>} */
@@ -410,9 +416,10 @@ export class Batch {
 
 						// TODO do we need to do anything with `target`? defer block effects?
 
-						queued_root_effects = [];
 						batch.deactivate();
 					}
+
+					queued_root_effects = prev_queued_root_effects;
 				}
 			}
 

packages/svelte/tests/runtime-runes/samples/async-block-effect-queueing/A.svelte

@@ -0,0 +1,15 @@
+<script>
+	let open = $state(false);
+	let menuOptionsEl = $state(null);
+</script>
+
+<button onclick={() => open = !open}>
+	toggle
+
+		{#if open}
+			<!-- bind:this uses effect, which is scheduled, causing queued_root_effects to be filled again -->
+			<span bind:this={menuOptionsEl}>
+				A
+			</span>
+		{/if}
+</button>

packages/svelte/tests/runtime-runes/samples/async-block-effect-queueing/B.svelte

@@ -0,0 +1 @@
+B

packages/svelte/tests/runtime-runes/samples/async-block-effect-queueing/_config.js

@@ -0,0 +1,63 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [fork, commit, toggle] = target.querySelectorAll('button');
+
+		fork.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>fork</button>
+			<button>commit</button>
+			<button>toggle</button>
+		`
+		);
+
+		toggle.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>fork</button>
+			<button>commit</button>
+			<button>toggle <span>A</span></button>
+		`
+		);
+
+		toggle.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>fork</button>
+			<button>commit</button>
+			<button>toggle</button>
+		`
+		);
+
+		toggle.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>fork</button>
+			<button>commit</button>
+			<button>toggle <span>A</span></button>
+		`
+		);
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>fork</button>
+			<button>commit</button>
+			B
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-block-effect-queueing/main.svelte

@@ -0,0 +1,24 @@
+<script lang="ts">
+	import { fork } from 'svelte';
+	import A from './A.svelte';
+	import B from './B.svelte';
+
+	let open = $state(true);
+	let f;
+</script>
+
+
+<button onclick={() => {
+	f = fork(() => {
+		open = !open;
+	})
+}}>fork</button>
+<button onclick={() => {
+	f.commit()
+}}>commit</button>
+
+{#if open}
+	<A />
+{:else}
+	<B />
+{/if}