fix: add hydration markers in pending branch of SSR boundary (#16965)

Author: paoloricciuti

.changeset/happy-numbers-stick.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: add hydration markers in `pending` branch of SSR boundary

packages/svelte/src/compiler/phases/3-transform/server/visitors/SvelteBoundary.js

@@ -49,7 +49,7 @@ export function SvelteBoundary(node, context) {
 			context.state.template.push(
 				b.if(
 					callee,
-					b.block([b.stmt(pending)]),
+					b.block(build_template([block_open_else, b.stmt(pending), block_close])),
 					b.block(build_template([block_open, statement, block_close]))
 				)
 			);

packages/svelte/tests/hydration/samples/boundary-pending-attribute/_config.js

@@ -0,0 +1,10 @@
+import { flushSync } from 'svelte';
+import { assert_ok, test } from '../../test';
+
+export default test({
+	compileOptions: {
+		experimental: {
+			async: true
+		}
+	}
+});

packages/svelte/tests/hydration/samples/boundary-pending-attribute/_expected.html

@@ -0,0 +1 @@
+<!--[--><!--[!--><!---->loading...<!--]--><!--]-->

packages/svelte/tests/hydration/samples/boundary-pending-attribute/main.svelte

@@ -0,0 +1,8 @@
+{#snippet pending()}
+	loading...
+{/snippet}
+
+<svelte:boundary {pending}>
+	{@const data = await Promise.resolve("data")}
+	{data}
+</svelte:boundary>