fix: don't mark deriveds as clean if updating during teardown (#15997)

Author: Rich-Harris

.changeset/old-insects-divide.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't mark deriveds as clean if updating during teardown

packages/svelte/src/internal/client/reactivity/deriveds.js

@@ -9,7 +9,8 @@ import {
 	update_reaction,
 	increment_write_version,
 	set_active_effect,
-	push_reaction_value
+	push_reaction_value,
+	is_destroying_effect
 } from '../runtime.js';
 import { equals, safe_equals } from './equality.js';
 import * as e from '../errors.js';
@@ -172,13 +173,18 @@ export function execute_derived(derived) {
  */
 export function update_derived(derived) {
 	var value = execute_derived(derived);
-	var status =
-		(skip_reaction || (derived.f & UNOWNED) !== 0) && derived.deps !== null ? MAYBE_DIRTY : CLEAN;
-
-	set_signal_status(derived, status);
 
 	if (!derived.equals(value)) {
 		derived.v = value;
 		derived.wv = increment_write_version();
 	}
+
+	// don't mark derived clean if we're reading it inside a
+	// cleanup function, or it will cache a stale value
+	if (is_destroying_effect) return;
+
+	var status =
+		(skip_reaction || (derived.f & UNOWNED) !== 0) && derived.deps !== null ? MAYBE_DIRTY : CLEAN;
+
+	set_signal_status(derived, status);
 }

packages/svelte/tests/runtime-runes/samples/effect-teardown-stale-value/_config.js

@@ -0,0 +1,22 @@
+import { test } from '../../test';
+import { flushSync } from 'svelte';
+
+export default test({
+	html: `<button>toggle (false)</button>`,
+
+	async test({ assert, target, logs }) {
+		assert.deepEqual(logs, ['up', { foo: false, bar: false }]);
+
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+		assert.deepEqual(logs, [
+			'up',
+			{ foo: false, bar: false },
+			'down',
+			{ foo: false, bar: false },
+			'up',
+			{ foo: true, bar: true }
+		]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/effect-teardown-stale-value/main.svelte

@@ -0,0 +1,14 @@
+<script>
+	let foo = $state(false)
+	let bar = $derived(foo)
+
+	$effect(() => {
+		console.log('up', { foo, bar });
+
+		return () =>{
+			console.log('down', { foo, bar });
+		};
+	});
+</script>
+
+<button onclick={() => foo = !foo}>toggle ({foo})</button>