fix: maintain reactivity for properties of deriveds with no deps

.changeset/old-pens-help.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: maintain reactivity for properties of deriveds with no deps

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

@@ -360,7 +360,10 @@ export function update_derived(derived) {
 		// the underlying value will be updated when the fork is committed.
 		// otherwise, the next time we get here after a 'real world' state
 		// change, `derived.equals` may incorrectly return `true`
-		if (!current_batch?.is_fork) {
+		//
+		// deriveds with no deps should always update `derived.v`
+		// since they will never change and need the value after fork commits
+		if (!current_batch?.is_fork || derived.deps === null) {
 			derived.v = value;
 		}
 
@@ -381,6 +384,11 @@ export function update_derived(derived) {
 		if (effect_tracking() || current_batch?.is_fork) {
 			batch_values.set(derived, value);
 		}
+		// For deriveds with no deps, set CLEAN to prevent re-evaluation
+		// since they can never become dirty from dependency changes
+		if (derived.deps === null) {
+			set_signal_status(derived, CLEAN);
+		}
 	} else {
 		var status = (derived.f & CONNECTED) === 0 ? MAYBE_DIRTY : CLEAN;
 		set_signal_status(derived, status);

packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/Child.svelte

@@ -0,0 +1,10 @@
+<script>
+	class Y {
+		foo = $state(0);
+	}
+
+	let y = $derived(new Y());
+</script>
+
+<button onclick={() => y.foo++}>click</button>
+<p>{y.foo}</p>

packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/_config.js

@@ -0,0 +1,24 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const forkButton = target.querySelector('button');
+
+		flushSync(() => {
+			forkButton?.click();
+		});
+
+		const [, clickButton] = target.querySelectorAll('button');
+		const p = target.querySelector('p');
+
+		assert.equal(p?.textContent, '0');
+
+		flushSync(() => {
+			clickButton?.click();
+		});
+
+		assert.equal(p?.textContent, '1');
+	}
+});

packages/svelte/tests/runtime-runes/samples/fork-derived-class-instance/main.svelte

@@ -0,0 +1,17 @@
+<script>
+	import { fork } from 'svelte';
+	import Child from './Child.svelte';
+
+	let x = $state(false);
+</script>
+
+<button onclick={() => {
+	const f = fork(() => {
+		x = true;
+	});
+	f.commit();
+}}>fork</button>
+
+{#if x}
+	<Child />
+{/if}