fix: improve handled of unowned derived signals

Author: trueadm

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

@@ -194,9 +194,21 @@ export function check_dirtiness(reaction) {
 				// is also dirty.
 				var version = dependency.version;
 
-				if (is_unowned && version > /** @type {import('#client').Derived} */ (reaction).version) {
-					/** @type {import('#client').Derived} */ (reaction).version = version;
-					return true;
+				if (is_unowned) {
+					if (version > /** @type {import('#client').Derived} */ (reaction).version) {
+						/** @type {import('#client').Derived} */ (reaction).version = version;
+						return true;
+					} else if (!current_skip_reaction && !dependency?.reactions?.includes(reaction)) {
+						// If we are working with an unowned signal as part of an effect (due to !current_skip_reaction)
+						// and the version hasn't changed, we still need to check that this reaction
+						// if linked to the dependency source – otherwise future updates will not be caught.
+						var reactions = dependency.reactions;
+						if (reactions === null) {
+							dependency.reactions = [reaction];
+						} else {
+							reactions.push(reaction);
+						}
+					}
 				}
 			}
 		}