fix: improve handled of unowned derived signals

Author: trueadm

.changeset/rich-garlics-laugh.md

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

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

@@ -0,0 +1,15 @@
+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>