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

@@ -205,29 +205,36 @@ export function check_dirtiness(reaction) {
 				reaction.f ^= DISCONNECTED;
 			}
 
+			var unowned_dirty = false;
+
 			for (i = 0; i < dependencies.length; i++) {
 				var dependency = dependencies[i];
 
-				if (check_dirtiness(/** @type {Derived} */ (dependency))) {
+				if (!unowned_dirty && check_dirtiness(/** @type {Derived} */ (dependency))) {
 					update_derived(/** @type {Derived} */ (dependency));
 				}
 
+				var version_mismatch = dependency.version > reaction.version;
+
 				// 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 (is_unowned && active_effect !== null && !skip_reaction) {
+					if (!dependency?.reactions?.includes(reaction)) {
+						(dependency.reactions ??= []).push(reaction);
+					}
 
-				if (dependency.version > reaction.version) {
+					if (version_mismatch) {
+						unowned_dirty = true;
+					}
+				} else if (version_mismatch) {
 					return true;
 				}
 			}
+
+			if (unowned_dirty) {
+				return true;
+			}
 		}
 
 		// Unowned signals should never be marked as clean unless they

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();
+		};
+	});
 });