fix: track missing proxy keys in unowned deriveds (#18944)

Co-authored-by: svelte-triage-bot <team@svelte.com>

.changeset/reactive-missing-property-checks.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: track missing state property checks in unowned deriveds

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

@@ -2,7 +2,6 @@
 import { DEV } from 'esm-env';
 import {
 	get,
-	active_effect,
 	update_version,
 	active_reaction,
 	set_update_version,
@@ -241,7 +240,7 @@ export function proxy(value) {
 
 			if (
 				s !== undefined ||
-				(active_effect !== null && (!has || get_descriptor(target, prop)?.writable))
+				(active_reaction !== null && (!has || get_descriptor(target, prop)?.writable))
 			) {
 				if (s === undefined) {
 					s = with_parent(() => {

packages/svelte/tests/signals/test.ts

@@ -153,6 +153,46 @@ describe('signals', () => {
 		};
 	});
 
+	test('unowned deriveds track missing property checks on proxies', () => {
+		const value = proxy<Record<string, boolean>>({});
+
+		return () => {
+			const has_x = derived(() => 'x' in value);
+			const owns_y = derived(() => Object.hasOwn(value, 'y'));
+
+			assert.isFalse($.get(has_x));
+			assert.isFalse($.get(owns_y));
+
+			value.x = true;
+			value.y = true;
+
+			assert.isTrue($.get(has_x));
+			assert.isTrue($.get(owns_y));
+
+			delete value.x;
+			delete value.y;
+
+			assert.isFalse($.get(has_x));
+			assert.isFalse($.get(owns_y));
+		};
+	});
+
+	test('unowned deriveds track missing array indexes', () => {
+		const value = proxy<boolean[]>([]);
+
+		return () => {
+			const has_first = derived(() => 0 in value);
+
+			assert.isFalse($.get(has_first));
+
+			value.push(true);
+			assert.isTrue($.get(has_first));
+
+			value.length = 0;
+			assert.isFalse($.get(has_first));
+		};
+	});
+
 	test('derived from state', () => {
 		const log: number[] = [];