Merge 20c955d5b1 into ce89035ecb

.changeset/calm-derived-reconnect.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent duplicate reactions when reconnecting deriveds

packages/svelte/src/internal/client/runtime.js

@@ -728,7 +728,10 @@ function reconnect(derived) {
 	if (derived.deps === null) return;
 
 	for (const dep of derived.deps) {
-		(dep.reactions ??= []).push(derived);
+		var reactions = (dep.reactions ??= []);
+		if (!includes.call(reactions, derived)) {
+			reactions.push(derived);
+		}
 
 		if ((dep.f & DERIVED) !== 0 && (dep.f & CONNECTED) === 0) {
 			unfreeze_derived_effects(/** @type {Derived} */ (dep));

packages/svelte/tests/signals/test.ts

@@ -810,6 +810,30 @@ describe('signals', () => {
 		};
 	});
 
+	test('deriveds with changed dependencies clean up after reconnection', () => {
+		const visible = state(true);
+		const first = state(0);
+		const second = state(1);
+		const selected = state(first);
+		const value = derived(() => $.get($.get(selected)));
+
+		return () => {
+			const destroy = effect_root(() => {
+				render_effect(() => {
+					if ($.get(visible)) $.get(value);
+				});
+			});
+
+			flushSync(() => set(visible, false));
+			flushSync(() => set(selected, second));
+			flushSync(() => set(visible, true));
+
+			destroy();
+
+			assert.equal(second.reactions, null);
+		};
+	});
+
 	test('deriveds update upon reconnection #1', () => {
 		let a = state(false);
 		let b = state(false);