fix: don't create async deriveds for a component destroyed while waiting for its top-level `await` (#18931)

Fixes #18930

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/brave-owls-wait.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't create async deriveds for a component destroyed while waiting for its top-level `await`

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

@@ -103,6 +103,11 @@ export function flatten(blockers, sync, async, fn) {
 
 	if (blocker_promise) {
 		blocker_promise.then(() => {
+			if ((parent.f & DESTROYED) !== 0) {
+				decrement_pending();
+				return;
+			}
+
 			restore();
 			run();
 			unset_context();

packages/svelte/tests/runtime-runes/samples/async-destroyed-before-blockers-settle/Child.svelte

@@ -0,0 +1,7 @@
+<script>
+	let { promise } = $props();
+
+	const value = await promise;
+</script>
+
+<p>{await Promise.resolve(value)}</p>

packages/svelte/tests/runtime-runes/samples/async-destroyed-before-blockers-settle/_config.js

@@ -0,0 +1,31 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [hide, resolve] = target.querySelectorAll('button');
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>hide</button>
+				<button>resolve</button>
+				<p>loading...</p>
+			`
+		);
+
+		hide.click();
+		await tick();
+
+		resolve.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>hide</button>
+				<button>resolve</button>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-destroyed-before-blockers-settle/main.svelte

@@ -0,0 +1,20 @@
+<script>
+	import Child from './Child.svelte';
+
+	const deferred = Promise.withResolvers();
+
+	let show = $state(true);
+</script>
+
+<button onclick={() => (show = false)}>hide</button>
+<button onclick={() => deferred.resolve(1)}>resolve</button>
+
+<svelte:boundary>
+	{#if show}
+		<Child promise={deferred.promise} />
+	{/if}
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>