fix: recover when an unexpected node replaces a nested boundary comment (#18925)

Fixes #18924


---------

Co-authored-by: nimbly1 <258366730+nimbly1@users.noreply.github.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/nested-boundary-hydrate.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: recover when an unexpected node replaces a nested boundary comment during hydration

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

@@ -32,6 +32,7 @@ import {
 	hydrate_node,
 	hydrating,
 	next,
+	read_hydration_instruction,
 	skip_nodes,
 	set_hydrate_node
 } from '../hydration.js';
@@ -164,16 +165,18 @@ export class Boundary {
 
 		this.#effect = block(() => {
 			if (hydrating) {
-				const comment = /** @type {Comment} */ (this.#hydrate_open);
+				const comment_data = read_hydration_instruction(
+					/** @type {TemplateNode} */ (this.#hydrate_open)
+				);
 				hydrate_next();
 
-				const server_rendered_pending = comment.data === HYDRATION_START_ELSE;
-				const server_rendered_failed = comment.data.startsWith(HYDRATION_START_FAILED);
+				const server_rendered_pending = comment_data === HYDRATION_START_ELSE;
+				const server_rendered_failed = comment_data.startsWith(HYDRATION_START_FAILED);
 
 				if (server_rendered_failed) {
 					// Server rendered the failed snippet - hydrate it.
 					// The serialized error is embedded in the comment: <!--[?<json>-->
-					const serialized_error = JSON.parse(comment.data.slice(HYDRATION_START_FAILED.length));
+					const serialized_error = JSON.parse(comment_data.slice(HYDRATION_START_FAILED.length));
 					this.#hydrate_failed_content(serialized_error);
 				} else if (server_rendered_pending) {
 					this.#hydrate_pending_content();

packages/svelte/tests/hydration/samples/boundary-nested-extra-node/_config.js

@@ -0,0 +1,6 @@
+import { test } from '../../test';
+
+// https://github.com/sveltejs/svelte/issues/18924
+export default test({
+	expect_hydration_error: true
+});

packages/svelte/tests/hydration/samples/boundary-nested-extra-node/_expected.html

@@ -0,0 +1 @@
+<div><p>inner</p></div>

packages/svelte/tests/hydration/samples/boundary-nested-extra-node/_override.html

@@ -0,0 +1 @@
+<!--[--><!--[--><div><span></span><!--[--><p>inner</p><!--]--></div><!--]--><!--]-->

packages/svelte/tests/hydration/samples/boundary-nested-extra-node/main.svelte

@@ -0,0 +1,14 @@
+<svelte:boundary>
+	<div>
+		<svelte:boundary>
+			<p>inner</p>
+			{#snippet failed()}
+				<p>inner failed</p>
+			{/snippet}
+		</svelte:boundary>
+	</div>
+
+	{#snippet failed()}
+		<p>outer failed</p>
+	{/snippet}
+</svelte:boundary>