fix: ensure async deriveds always get dependencies from thennable

.changeset/orange-chefs-float.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure async deriveds always get dependencies from thennable

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

@@ -120,6 +120,9 @@ export function async_derived(fn, location) {
 
 		try {
 			var p = fn();
+			// Make sure to always access the then property to read any signals
+			// it might access, so that we track them as dependencies.
+			if (prev) Promise.resolve(p).catch(() => {}); // avoid unhandled rejection
 		} catch (error) {
 			p = Promise.reject(error);
 		}

packages/svelte/tests/runtime-runes/samples/async-derived-reverse-order/_config.js

@@ -0,0 +1,41 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const [increment, pop] = target.querySelectorAll('button');
+
+		increment.click();
+		await tick();
+
+		pop.click();
+		await tick();
+
+		pop.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>increment</button>
+				<button>pop</button>
+				<p>1</p>
+			`
+		);
+
+		increment.click();
+		await tick();
+
+		pop.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>increment</button>
+				<button>pop</button>
+				<p>2</p>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-derived-reverse-order/main.svelte

@@ -0,0 +1,35 @@
+
+<script>
+	let count = $state(0);
+
+	let deferreds = [];
+
+	class X {
+		constructor(promise) {
+			this.promise = promise;
+		}
+
+		get then() {
+			count;
+
+			return (resolve) => this.promise.then(() => count).then(resolve)
+		}
+	}
+
+	function push() {
+		const deferred = Promise.withResolvers();
+		deferreds.push(deferred);
+		return new X(deferred.promise);
+	}
+</script>
+
+<button onclick={() => count += 1}>increment</button>
+<button onclick={() => deferreds.pop()?.resolve(count)}>pop</button>
+
+<svelte:boundary>
+	<p>{await push()}</p>
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>