fix: ensure deferred effects can be rescheduled later on

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 https://github.com/sveltejs/svelte/issues/17118#issuecomment-3521488865

.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';
@@ -270,11 +271,32 @@ 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>