fix: prevent hydration error on async {@html ...} (#17999)

Fixes #17981

Author: dummdidumm

.changeset/four-beers-like.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent hydration error on async `{@html ...}`

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/fragment.js

@@ -101,16 +101,14 @@ export function process_children(nodes, initial, is_element, context) {
 			if (is_static_element(node)) {
 				skipped += 1;
 			} else if (
-				node.type === 'EachBlock' &&
+				(node.type === 'EachBlock' || node.type === 'HtmlTag') &&
 				nodes.length === 1 &&
 				is_element &&
 				// In case it's wrapped in async the async logic will want to skip sibling nodes up until the end, hence we cannot make this controlled
 				// TODO switch this around and instead optimize for elements with a single block child and not require extra comments (neither for async nor normally)
 				!node.metadata.expression.is_async()
 			) {
 				node.metadata.is_controlled = true;
-			} else if (node.type === 'HtmlTag' && nodes.length === 1 && is_element) {
-				node.metadata.is_controlled = true;
 			} else {
 				const id = flush_node(
 					false,

packages/svelte/tests/runtime-runes/samples/async-hydrate-html-tag/_config.js

@@ -0,0 +1,10 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['hydrate'],
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, `<div><div><p>first test</p></div> other test</div>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-hydrate-html-tag/main.svelte

@@ -0,0 +1,14 @@
+<script>
+	function firstTest() {
+		return Promise.resolve('<p>first test</p>');
+	}
+
+	function otherTest() {
+		return Promise.resolve('other test');
+	}
+</script>
+
+<div>
+	<div>{@html await firstTest()}</div>
+	{await otherTest()}
+</div>