fix: improve handled of unowned derived signals

.changeset/rich-garlics-laugh.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: improve handled of unowned derived signals

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);
+						}
+					}
 				}
 			}
 		}

packages/svelte/tests/runtime-runes/samples/derived-unowned-5/_config.js

@@ -0,0 +1,12 @@
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		// The test has a bunch of queueMicrotasks
+		await Promise.resolve();
+		await Promise.resolve();
+		await Promise.resolve();
+
+		assert.htmlEqual(target.innerHTML, `<div>Zeeba Neighba</div>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/derived-unowned-5/main.svelte

@@ -0,0 +1,34 @@
+<script context="module">
+  export class Thing {
+    data = $state();
+
+    subscribe() {
+      queueMicrotask(() => {
+        this.data = {
+          name: `Zeeba Neighba`,
+        };
+      });
+    }
+
+    name = $derived(this.data?.name);
+  }
+
+  export class Things {
+    thing = $state();
+
+    subscribe() {
+      queueMicrotask(() => {
+        this.thing = new Thing();
+        this.thing.subscribe();
+        this.thing.name;
+      });
+    }
+  }
+</script>
+
+<script>
+  let model = new Things();
+  $effect(() => model.subscribe());
+</script>
+
+<div>{model.thing?.name}</div>