fix: advance hydration before skipping settled async blocks (#17539)

* fix: advance hydration before skipping settled async blocks

* add test sample

* simplify

.changeset/common-parrots-do.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: hydration failing with settled async blocks

packages/svelte/src/internal/client/dom/blocks/async.js

@@ -19,6 +19,12 @@ import { get_boundary } from './boundary.js';
  * @param {(anchor: TemplateNode, ...deriveds: Value[]) => void} fn
  */
 export function async(node, blockers = [], expressions = [], fn) {
+	var was_hydrating = hydrating;
+
+	if (was_hydrating) {
+		hydrate_next();
+	}
+
 	if (expressions.length === 0 && blockers.every((b) => b.settled)) {
 		fn(node);
 		return;
@@ -31,11 +37,7 @@ export function async(node, blockers = [], expressions = [], fn) {
 	boundary.update_pending_count(1);
 	batch.increment(blocking);
 
-	var was_hydrating = hydrating;
-
 	if (was_hydrating) {
-		hydrate_next();
-
 		var previous_hydrate_node = hydrate_node;
 		var end = skip_nodes(false);
 		set_hydrate_node(end);

packages/svelte/tests/runtime-runes/samples/async-settled-blockers-hydration/_config.js

@@ -0,0 +1,14 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	skip_mode: ['server'],
+	skip_no_async: true,
+
+	ssrHtml: `<div>1</div><p>after</p>`,
+
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<div>1</div><p>after</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-settled-blockers-hydration/main.svelte

@@ -0,0 +1,13 @@
+<script>
+	const n = $derived(await Promise.resolve(1));
+</script>
+
+{#snippet child(n)}
+	<div>{n}</div>
+{/snippet}
+
+{#if n}
+	{@render child(n)}
+{/if}
+
+<p>after</p>