fix: ensure scheduled batch is flushed if not obsolete (#18131)

The logic of checking that the current batch is still the generated one
is flawed. If microtasks align the current batch can be a different
value even if the batch still need to be flushed. This therefore
switches the heuristic to what it actually should express: "has this
batch run already?"

Fixes #18126 (because the batch isn't running, it later runs into the
invariant)

.changeset/many-pandas-add.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure scheduled batch is flushed if not obsolete

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

@@ -716,7 +716,7 @@ export class Batch {
 
 				if (!is_flushing_sync) {
 					queue_micro_task(() => {
-						if (current_batch !== batch) {
+						if (!batches.has(batch) || batch.#pending.size > 0) {
 							// a flushSync happened in the meantime
 							return;
 						}

packages/svelte/tests/runtime-runes/samples/async-state-updates-microtask-separated/_config.js

@@ -0,0 +1,15 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+// Ensure that microtask timing doesn't influence whether or not a scheduled batch is flushed.
+// Timing can be such that the current_batch is reset before the scheduled flush runs, which
+// would cause the flush to skip without the fix.
+export default test({
+	async test({ assert, target }) {
+		const [btn] = target.querySelectorAll('button');
+
+		btn.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '1 1');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-state-updates-microtask-separated/main.svelte

@@ -0,0 +1,18 @@
+<script>
+	let a = $state(0);
+	let b = $state(0);
+</script>
+
+{#if a}
+	{@const toShow = await a}
+	{toShow}
+	{b}
+{:else}
+	<button
+		onclick={async () => {
+			a = 1;
+			await 1;
+			await 1; // two microtasks needed to get timing right to reproduce the bug
+			b = 1;
+		}}>click</button>
+{/if}