chore: test for batches flush fix

packages/svelte/tests/runtime-runes/samples/async-inner-after-outer/_config.js

@@ -0,0 +1,57 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const shift = document.querySelector('button');
+		shift?.click();
+		await tick();
+		shift?.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<p>true</p>
+			<button>toggle</button>
+			<button>shift</button>
+		`
+		);
+
+		const toggle = target.querySelector('button');
+		toggle?.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<p>true</p>
+			<button>toggle</button>
+			<button>shift</button>
+		`
+		);
+
+		shift?.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<p>true</p>
+			<button>toggle</button>
+			<button>shift</button>
+		`
+		);
+
+		shift?.click();
+		await tick();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<button>toggle</button>
+			<button>shift</button>
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-inner-after-outer/main.svelte

@@ -0,0 +1,41 @@
+<script>
+	let show = $state(true);
+	let count = $state(0);
+	let queue = [];
+
+	function foo() {
+		const {promise, resolve} = Promise.withResolvers();
+		const s = show;
+		queue.push(() => resolve(s));
+		return promise;
+	}
+
+	function bar() {
+		const {promise, resolve} = Promise.withResolvers();
+		const s = show;
+		queue.push(() => {
+			// This will create a new batch while the other batch is still in flight
+			count++
+			resolve(s);
+		});
+		return promise;
+	}
+
+	$effect(() => { count; });
+</script>
+
+<svelte:boundary>
+	{#if await foo()}
+		<p>{await bar()}</p>
+	{/if}
+
+	<button onclick={() => {
+		show = !show
+	}}>toggle</button>
+
+	{#snippet pending()}
+		<p>loading...</p>
+	{/snippet}
+</svelte:boundary>
+
+<button onclick={() => queue.shift()()}>shift</button>