fix: prevent reactivity loss during/after fork

.changeset/common-boats-travel.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent reactivity loss during fork

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

@@ -110,6 +110,8 @@ export class Batch {
 	 */
 	#blocking_pending = 0;
 
+	#decrement_queued = false;
+
 	/**
 	 * A deferred that resolves when the batch is committed, used with `settled()`
 	 * TODO replace with Promise.withResolvers once supported widely enough
@@ -138,7 +140,11 @@ export class Batch {
 
 	is_fork = false;
 
-	#decrement_queued = false;
+	/**
+	 * Branches that had their CLEAN flag toggled during fork execution
+	 * @type {Set<Effect> | null}
+	 */
+	toggled_branches = null;
 
 	is_deferred() {
 		return this.is_fork || this.#blocking_pending > 0;
@@ -816,6 +822,12 @@ export function schedule_effect(signal) {
 		if ((flags & (ROOT_EFFECT | BRANCH_EFFECT)) !== 0) {
 			if ((flags & CLEAN) === 0) return;
 			effect.f ^= CLEAN;
+
+			// Track branches toggled during fork execution so we can restore
+			// their CLEAN flag after flush
+			if (current_batch !== null && current_batch.is_fork) {
+				(current_batch.toggled_branches ??= new Set()).add(effect);
+			}
 		}
 	}
 
@@ -918,6 +930,16 @@ export function fork(fn) {
 
 	flushSync(fn);
 
+	// Restore CLEAN flags that were toggled during fork initialization.
+	if (batch.toggled_branches !== null) {
+		for (const effect of batch.toggled_branches) {
+			effect.f |= CLEAN;
+		}
+		batch.toggled_branches = null;
+	}
+
+	batch_values = null;
+
 	// revert state changes
 	for (var [source, value] of batch.previous) {
 		source.v = value;

packages/svelte/tests/runtime-runes/samples/fork-discard-reactivity/Child.svelte

@@ -0,0 +1,5 @@
+<script>
+	let count = $state(0);
+</script>
+
+<button onclick={() => count++}>{count}</button>

packages/svelte/tests/runtime-runes/samples/fork-discard-reactivity/_config.js

@@ -0,0 +1,21 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const [fork_btn, counter_btn] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			fork_btn.click();
+		});
+
+		assert.equal(counter_btn.textContent, '0');
+
+		flushSync(() => {
+			counter_btn.click();
+		});
+
+		assert.equal(counter_btn.textContent, '1');
+	}
+});

packages/svelte/tests/runtime-runes/samples/fork-discard-reactivity/main.svelte

@@ -0,0 +1,21 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let show = $state(false);
+</script>
+
+<button onclick={() => {
+	let f = fork(() => {
+		show = !show;
+	});
+	f.discard();
+}}>fork</button>
+
+{#if show}
+	hi
+{:else}
+	{#if show || !show}
+		<Child />
+	{/if}
+{/if}

packages/svelte/tests/runtime-runes/samples/fork-pending-reactivity/Child.svelte

@@ -0,0 +1,5 @@
+<script>
+	let count = $state(0);
+</script>
+
+<button onclick={() => count++}>{count}</button>

packages/svelte/tests/runtime-runes/samples/fork-pending-reactivity/_config.js

@@ -0,0 +1,27 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const [fork_btn, counter_btn] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			fork_btn.click();
+		});
+
+		assert.equal(counter_btn.textContent, '0');
+
+		flushSync(() => {
+			counter_btn.click();
+		});
+
+		assert.equal(counter_btn.textContent, '1');
+
+		flushSync(() => {
+			counter_btn.click();
+		});
+
+		assert.equal(counter_btn.textContent, '2');
+	}
+});

packages/svelte/tests/runtime-runes/samples/fork-pending-reactivity/main.svelte

@@ -0,0 +1,22 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let show = $state(false);
+	let pendingFork = $state(null);
+</script>
+
+<button onclick={() => {
+	// Create fork but don't discard to simulate SvelteKit preload
+	pendingFork = fork(() => {
+		show = !show;
+	});
+}}>fork</button>
+
+{#if show}
+	hi
+{:else}
+	{#if show || !show}
+		<Child />
+	{/if}
+{/if}