fix: keep deriveds reactive after their original parent effect was destroyed

.changeset/sad-forks-go.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: keep deriveds reactive after their original parent effect was destroyed

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

@@ -11,7 +11,8 @@ import {
 	STALE_REACTION,
 	ASYNC,
 	WAS_MARKED,
-	CONNECTED
+	CONNECTED,
+	DESTROYED
 } from '#client/constants';
 import {
 	active_reaction,
@@ -296,7 +297,9 @@ function get_derived_parent_effect(derived) {
 	var parent = derived.parent;
 	while (parent !== null) {
 		if ((parent.f & DERIVED) === 0) {
-			return /** @type {Effect} */ (parent);
+			// The original parent effect might've been destroyed but the derived
+			// is used elsewhere now - do not return the destroyed effect in that case
+			return (parent.f & DESTROYED) === 0 ? /** @type {Effect} */ (parent) : null;
 		}
 		parent = parent.parent;
 	}

packages/svelte/tests/signals/test.ts

@@ -1391,6 +1391,33 @@ describe('signals', () => {
 		};
 	});
 
+	test('derived whose original parent effect has been destroyed keeps updating', () => {
+		return () => {
+			let count: Source<number>;
+			let double: Derived<number>;
+			const destroy = effect_root(() => {
+				render_effect(() => {
+					count = state(0);
+					double = derived(() => $.get(count) * 2);
+				});
+			});
+
+			flushSync();
+			assert.equal($.get(double!), 0);
+
+			destroy();
+			flushSync();
+
+			set(count!, 1);
+			flushSync();
+			assert.equal($.get(double!), 2);
+
+			set(count!, 2);
+			flushSync();
+			assert.equal($.get(double!), 4);
+		};
+	});
+
 	test('$effect.root inside deriveds stay alive independently', () => {
 		const log: any[] = [];
 		const c = state(0);