fix: exit resolved async blocks on correct node when hydrating (#17640)

* fix: exit resolved async blocks on correct node when hydrating

* expand test + fix

* tweak, add note to self

---------

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

.changeset/tiny-owls-pay.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: exit resolved async blocks on correct node when hydrating

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

@@ -20,13 +20,27 @@ import { get_boundary } from './boundary.js';
  */
 export function async(node, blockers = [], expressions = [], fn) {
 	var was_hydrating = hydrating;
+	var end = null;
 
 	if (was_hydrating) {
 		hydrate_next();
+		end = skip_nodes(false);
 	}
 
 	if (expressions.length === 0 && blockers.every((b) => b.settled)) {
 		fn(node);
+
+		// This is necessary because it is not guaranteed that the render function will
+		// advance the hydration node to $.async's end marker: it may stop at an inner
+		// block's end marker (in case of an inner if block for example), but it also may
+		// stop at the correct $.async end marker (in case of component child) - hence
+		// we can't just use hydrate_next()
+		// TODO this feels indicative of a bug elsewhere; ideally we wouldn't need
+		// to double-traverse in the already-resolved case
+		if (was_hydrating) {
+			set_hydrate_node(end);
+		}
+
 		return;
 	}
 
@@ -39,7 +53,6 @@ export function async(node, blockers = [], expressions = [], fn) {
 
 	if (was_hydrating) {
 		var previous_hydrate_node = hydrate_node;
-		var end = skip_nodes(false);
 		set_hydrate_node(end);
 	}
 

packages/svelte/tests/runtime-runes/samples/async-if-hydration/Child.svelte

@@ -0,0 +1,5 @@
+<script>
+    let { b } = $props();
+</script>
+
+{b}

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

@@ -0,0 +1,11 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['hydrate'],
+
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<div><p>hello</p></div> <div>true</div>`);
+	}
+});

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

@@ -0,0 +1,18 @@
+<script lang="ts">
+	import Child from "./Child.svelte";
+
+	await 1;
+	let b = true;
+	let a = true;
+</script>
+
+{#if a}
+	<div>
+		{#if b}
+			<p>hello</p>
+		{/if}
+	</div>
+	<div>
+		<Child {b} />
+	</div>
+{/if}