fix: ensure unowned deriveds correctly get re-linked to the graph

.changeset/chatty-dolphins-complain.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure unowned deriveds correctly get re-linked to the graph

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

@@ -196,34 +196,34 @@ export function check_dirtiness(reaction) {
 
 		if (dependencies !== null) {
 			var i;
-
-			if ((flags & DISCONNECTED) !== 0) {
-				for (i = 0; i < dependencies.length; i++) {
-					(dependencies[i].reactions ??= []).push(reaction);
+			var dependency;
+			var is_disconnected = (flags & DISCONNECTED) !== 0;
+			var is_unowned_connected = is_unowned && active_effect !== null && !skip_reaction;
+			var length = dependencies.length;
+
+			// If we are working with a disconnected or an unowned signal that is now connected (due to an active effect)
+			// then we need to re-connect the reaction to the dependency
+			if (is_disconnected || is_unowned_connected) {
+				for (i = 0; i < length; i++) {
+					dependency = dependencies[i];
+
+					if (!dependency?.reactions?.includes(reaction)) {
+						(dependency.reactions ??= []).push(reaction);
+					}
 				}
 
-				reaction.f ^= DISCONNECTED;
+				if (is_disconnected) {
+					reaction.f ^= DISCONNECTED;
+				}
 			}
 
-			for (i = 0; i < dependencies.length; i++) {
-				var dependency = dependencies[i];
+			for (i = 0; i < length; i++) {
+				dependency = dependencies[i];
 
 				if (check_dirtiness(/** @type {Derived} */ (dependency))) {
 					update_derived(/** @type {Derived} */ (dependency));
 				}
 
-				// If we are working with an unowned signal as part of an effect (due to !skip_reaction)
-				// and the version hasn't changed, we still need to check that this reaction
-				// is linked to the dependency source – otherwise future updates will not be caught.
-				if (
-					is_unowned &&
-					active_effect !== null &&
-					!skip_reaction &&
-					!dependency?.reactions?.includes(reaction)
-				) {
-					(dependency.reactions ??= []).push(reaction);
-				}
-
 				if (dependency.version > reaction.version) {
 					return true;
 				}

packages/svelte/tests/signals/test.ts

@@ -781,4 +781,37 @@ describe('signals', () => {
 			assert.equal($.get(count), 0n);
 		};
 	});
+
+	test('unowned deriveds correctly re-attach to their source', () => {
+		const log: any[] = [];
+
+		return () => {
+			const a = state(0);
+			const b = state(0);
+			const c = derived(() => {
+				$.get(a);
+				return $.get(b);
+			});
+
+			$.get(c);
+
+			set(a, 1);
+
+			const destroy = effect_root(() => {
+				render_effect(() => {
+					log.push($.get(c));
+				});
+			});
+
+			assert.deepEqual(log, [0]);
+
+			set(b, 1);
+
+			flushSync();
+
+			assert.deepEqual(log, [0, 1]);
+
+			destroy();
+		};
+	});
 });