fix: prevent derives without dependencies from ever re-running (#17445)

* fix: prevent derives without dependencies from ever re-running

* tweak

Co-authored-by: David Roizenman <hmnd@users.noreply.github.com>

.changeset/plenty-candies-notice.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent derives without dependencies from ever re-running

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

@@ -9,7 +9,8 @@ import {
 	STALE_REACTION,
 	ASYNC,
 	WAS_MARKED,
-	DESTROYED
+	DESTROYED,
+	CLEAN
 } from '#client/constants';
 import {
 	active_reaction,
@@ -33,7 +34,7 @@ import { UNINITIALIZED } from '../../../constants.js';
 import { batch_values, current_batch } from './batch.js';
 import { unset_context } from './async.js';
 import { deferred } from '../../shared/utils.js';
-import { update_derived_status } from './status.js';
+import { set_signal_status, update_derived_status } from './status.js';
 
 /** @type {Effect | null} */
 export let current_async_effect = null;
@@ -356,15 +357,21 @@ export function update_derived(derived) {
 	var value = execute_derived(derived);
 
 	if (!derived.equals(value)) {
+		derived.wv = increment_write_version();
+
 		// in a fork, we don't update the underlying value, just `batch_values`.
 		// 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) {
+		if (!current_batch?.is_fork || derived.deps === null) {
 			derived.v = value;
-		}
 
-		derived.wv = increment_write_version();
+			// deriveds without dependencies should never be recomputed
+			if (derived.deps === null) {
+				set_signal_status(derived, CLEAN);
+				return;
+			}
+		}
 	}
 
 	// don't mark derived clean if we're reading it inside a

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

@@ -0,0 +1,22 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_no_async: true,
+	async test({ assert, target }) {
+		const [fork] = target.querySelectorAll('button');
+
+		fork.click();
+		await tick();
+
+		const [, increment] = target.querySelectorAll('button');
+		const p = target.querySelector('p');
+
+		assert.equal(p?.textContent, '0');
+
+		increment.click();
+		await tick();
+
+		assert.equal(p?.textContent, '1');
+	}
+});

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

@@ -0,0 +1,23 @@
+<script>
+	import { fork } from 'svelte';
+
+	class Counter {
+		count = $state(0);
+	}
+
+	let condition = $state(false);
+	let counter = $derived(new Counter());
+</script>
+
+<button onclick={() => {
+	fork(() => {
+		condition = true;
+	}).commit();
+}}>fork</button>
+
+{#if condition}
+	<button onclick={() => {
+		counter.count++;
+	}}>click</button>
+	<p>{counter.count}</p>
+{/if}