fix: improve unowned derived signal heuristics (#11029)

* fix: improve unowned derived signal heuristics

* spelling

.changeset/big-moons-occur.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: improve unowned derived signal heuristics

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

@@ -171,6 +171,7 @@ export function check_dirtiness(reaction) {
 
 	if ((flags & MAYBE_DIRTY) !== 0) {
 		var dependencies = reaction.deps;
+		var is_unowned = (flags & UNOWNED) !== 0;
 
 		if (dependencies !== null) {
 			var length = dependencies.length;
@@ -191,7 +192,6 @@ export function check_dirtiness(reaction) {
 				// if our dependency write version is higher. If it is then we can assume
 				// that state has changed to a newer version and thus this unowned signal
 				// is also dirty.
-				var is_unowned = (flags & UNOWNED) !== 0;
 				var version = dependency.version;
 
 				if (is_unowned && version > /** @type {import('#client').Derived} */ (reaction).version) {
@@ -201,7 +201,10 @@ export function check_dirtiness(reaction) {
 			}
 		}
 
-		set_signal_status(reaction, CLEAN);
+		// Unowned signals are always maybe dirty, as we instead check their dependency versions.
+		if (!is_unowned) {
+			set_signal_status(reaction, CLEAN);
+		}
 	}
 
 	return false;

packages/svelte/tests/runtime-runes/samples/derived-unowned-4/_config.js

@@ -0,0 +1,17 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		let [btn1] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			btn1.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>update</button><div>2</div><div>2</div><div>2</div><div>2</div>`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/derived-unowned-4/main.svelte

@@ -0,0 +1,33 @@
+<script>
+  let thing = $state();
+
+	function update() {
+		let data = $state({ name: 1, position: 1 });
+	  let position = $derived(data.position);
+		let name = $derived(data.name);
+
+		thing = {
+			get data() { return data },
+			get position() { return position },
+			get name() { return name },
+		}
+
+		thing.position;
+		data = { name: 2, position: 2 };
+	}
+</script>
+
+<button onclick={update}>update</button>
+
+<div>
+	{thing?.data?.name}
+</div>
+<div>
+	{thing?.name}
+</div>
+<div>
+	{thing?.data?.position}
+</div>
+<div>
+	{thing?.position}
+</div>