fix: ensure unowned deriveds can add themselves as reactions while connected (#16249)

an unowned derived should still add itself as a reaction while it is properly connected, which means it can be cleaned up correctly - and this connection is indicated by it having reactions.

Fixes #15829 and potentially #15853

Author: dummdidumm

.changeset/little-kings-smoke.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure unowned deriveds can add themselves as reactions while connected

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

@@ -294,7 +294,12 @@ export function update_reaction(reaction) {
 				reaction.deps = deps = new_deps;
 			}
 
-			if (!skip_reaction) {
+			if (
+				!skip_reaction ||
+				// Deriveds that already have reactions can cleanup, so we still add them as reactions
+				((flags & DERIVED) !== 0 &&
+					/** @type {import('#client').Derived} */ (reaction).reactions !== null)
+			) {
 				for (i = skipped_deps; i < deps.length; i++) {
 					(deps[i].reactions ??= []).push(reaction);
 				}

packages/svelte/tests/signals/test.ts

@@ -112,6 +112,45 @@ describe('signals', () => {
 		};
 	});
 
+	test('unowned deriveds are not added as reactions but trigger effects', () => {
+		var obj = state<any>(undefined);
+
+		class C1 {
+			#v = state(0);
+			get v() {
+				return $.get(this.#v);
+			}
+			set v(v: number) {
+				set(this.#v, v);
+			}
+		}
+
+		return () => {
+			let d = derived(() => $.get(obj)?.v || '-');
+
+			const log: number[] = [];
+			assert.equal($.get(d), '-');
+
+			let destroy = effect_root(() => {
+				render_effect(() => {
+					log.push($.get(d));
+				});
+			});
+
+			set(obj, new C1());
+			flushSync();
+			assert.equal($.get(d), '-');
+			$.get(obj).v = 1;
+			flushSync();
+			assert.equal($.get(d), 1);
+			assert.deepEqual(log, ['-', 1]);
+			destroy();
+			// ensure we're not leaking reactions
+			assert.equal(obj.reactions, null);
+			assert.equal(d.reactions, null);
+		};
+	});
+
 	test('derived from state', () => {
 		const log: number[] = [];