fix: ensure deferred effects can be rescheduled later on (#17147)

When deferring effects we didn't unmark the deriveds that lead to those effects. This means that they might not be reached in subsequent runs of `mark_reactions`.

Fixes #17118 (comment)

Author: dummdidumm

.changeset/grumpy-gifts-sit.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure deferred effects can be rescheduled later on

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

@@ -16,7 +16,8 @@ import {
 	BOUNDARY_EFFECT,
 	EAGER_EFFECT,
 	HEAD_EFFECT,
-	ERROR_VALUE
+	ERROR_VALUE,
+	WAS_MARKED
 } from '#client/constants';
 import { async_mode_flag } from '../../flags/index.js';
 import { deferred, define_property } from '../../shared/utils.js';
@@ -274,11 +275,32 @@ export class Batch {
 			const target = (e.f & DIRTY) !== 0 ? this.#dirty_effects : this.#maybe_dirty_effects;
 			target.push(e);
 
+			// Since we're not executing these effects now, we need to clear any WAS_MARKED flags
+			// so that other batches can correctly reach these effects during their own traversal
+			this.#clear_marked(e.deps);
+
 			// mark as clean so they get scheduled if they depend on pending async state
 			set_signal_status(e, CLEAN);
 		}
 	}
 
+	/**
+	 * @param {Value[] | null} deps
+	 */
+	#clear_marked(deps) {
+		if (deps === null) return;
+
+		for (const dep of deps) {
+			if ((dep.f & DERIVED) === 0 || (dep.f & WAS_MARKED) === 0) {
+				continue;
+			}
+
+			dep.f ^= WAS_MARKED;
+
+			this.#clear_marked(/** @type {Derived} */ (dep).deps);
+		}
+	}
+
 	/**
 	 * Associate a change to a given source with the current
 	 * batch, noting its previous and current values

packages/svelte/tests/runtime-runes/samples/async-fork-update-same-state/_config.js

@@ -0,0 +1,49 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		assert.deepEqual(logs, [0]);
+
+		const [fork1, fork2, commit] = target.querySelectorAll('button');
+
+		fork1.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>fork 1</button>
+			<button>fork 2</button>
+			<button>commit</button>
+			<p>0</p>
+		`
+		);
+		assert.deepEqual(logs, [0]);
+
+		fork2.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>fork 1</button>
+			<button>fork 2</button>
+			<button>commit</button>
+			<p>0</p>
+		`
+		);
+		assert.deepEqual(logs, [0]);
+
+		commit.click();
+		await tick();
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>fork 1</button>
+			<button>fork 2</button>
+			<button>commit</button>
+			<p>1</p>
+		`
+		);
+		assert.deepEqual(logs, [0, 1]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-update-same-state/main.svelte

@@ -0,0 +1,37 @@
+<script>
+	import { fork } from "svelte";
+	
+	let state = $state(0);
+
+	let count = $derived(state);
+
+	$effect.pre(() => {
+		console.log(count);
+	});
+
+	let forked;
+</script>
+
+<button onclick={()=>{
+	forked?.discard?.();
+	forked = fork(()=>{
+		state++;
+	});
+}}>
+	fork 1
+</button>
+
+<button onclick={()=>{
+	forked?.discard?.();
+	forked = fork(()=>{
+		state++;
+	})
+}}>
+	fork 2
+</button>
+
+<button onclick={()=>{
+	forked?.commit();
+}}>commit</button>
+
+<p>{count}</p>