fix: ensure reactions are correctly attached for unowned deriveds (#15158)

* fix: ensure reactions are correctly attached for unowned deriveds

* tune

.changeset/loud-cars-scream.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure reactions are correctly attached for unowned deriveds

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

@@ -802,12 +802,19 @@ function process_effects(effect, collected_effects) {
 				if (is_branch) {
 					current_effect.f ^= CLEAN;
 				} else {
+					// Ensure we set the effect to be the active reaction
+					// to ensure that unowned deriveds are correctly tracked
+					// because we're flushing the current effect
+					var previous_active_reaction = active_reaction;
 					try {
+						active_reaction = current_effect;
 						if (check_dirtiness(current_effect)) {
 							update_effect(current_effect);
 						}
 					} catch (error) {
 						handle_error(error, current_effect, null, current_effect.ctx);
+					} finally {
+						active_reaction = previous_active_reaction;
 					}
 				}
 
@@ -952,15 +959,13 @@ export function get(signal) {
 		var derived = /** @type {Derived} */ (signal);
 		var parent = derived.parent;
 
-		if (parent !== null) {
+		if (parent !== null && (parent.f & UNOWNED) === 0) {
 			// If the derived is owned by another derived then mark it as unowned
 			// as the derived value might have been referenced in a different context
 			// since and thus its parent might not be its true owner anymore
-			if ((parent.f & UNOWNED) === 0) {
 			derived.f ^= UNOWNED;
 		}
 	}
-	}
 
 	if (is_derived) {
 		derived = /** @type {Derived} */ (signal);

packages/svelte/tests/runtime-runes/samples/derived-unowned-11/Child.svelte

@@ -0,0 +1,5 @@
+<script>
+	let { value = $bindable() } = $props()
+</script>
+
+<button onclick={() => value = 'a'}>change</button>

packages/svelte/tests/runtime-runes/samples/derived-unowned-11/Child2.svelte

@@ -0,0 +1,5 @@
+<script>
+	let {disabled = false} = $props()
+</script>
+
+{disabled}

packages/svelte/tests/runtime-runes/samples/derived-unowned-11/_config.js

@@ -0,0 +1,16 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		let [btn1, btn2] = target.querySelectorAll('button');
+
+		btn1?.click();
+		flushSync();
+
+		btn2?.click();
+		flushSync();
+
+		assert.htmlEqual(target.innerHTML, `<button>change</button><button>change</button>\nfalse`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/derived-unowned-11/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import Child2 from './Child2.svelte'
+	import Child from './Child.svelte'
+	let loginname = $state('')
+	let password = $state('')
+</script>
+
+<Child bind:value={loginname} />
+<Child bind:value={password} />
+<Child2 disabled={!loginname || !password} />
+