fix: skip single node optimization in custom renderer

packages/svelte/src/compiler/phases/3-transform/utils.js

@@ -12,6 +12,7 @@ import { extract_identifiers } from '../../utils/ast.js';
 import check_graph_for_cycles from '../2-analyze/utils/check_graph_for_cycles.js';
 import is_reference from 'is-reference';
 import { set_scope } from '../scope.js';
+import { custom_renderer } from '../../state.js';
 
 /**
  * Match Svelte 4 behaviour by sorting ConstTag nodes in topological order
@@ -290,7 +291,8 @@ export function clean_nodes(
 					!first.metadata.dynamic &&
 					!first.attributes.some(
 						(attribute) => attribute.type === 'Attribute' && attribute.name.startsWith('--')
-					))),
+					))) &&
+			custom_renderer == undefined,
 		/** if a component/snippet/each block starts with text, we need to add an anchor comment so that its text node doesn't get fused with its surroundings */
 		is_text_first:
 			(parent.type === 'Fragment' ||

packages/svelte/tests/snapshot/samples/custom-renderer-single-node/Component.svelte

@@ -0,0 +1,5 @@
+<script lang="ts">
+	let { children } = $props();
+</script>
+
+{@render children?.()}

packages/svelte/tests/snapshot/samples/custom-renderer-single-node/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		experimental: {
+			// The actual renderer module is irrelevant for this snapshot — we only
+			// care that the server output is a no-op while the client output still
+			// imports/uses the custom renderer. Using a fixed path keeps the
+			// snapshot stable across machines.
+			customRenderer: 'my-custom-renderer'
+		}
+	}
+});

packages/svelte/tests/snapshot/samples/custom-renderer-single-node/_expected/client/Component.svelte.js

@@ -0,0 +1,13 @@
+import $renderer from 'my-custom-renderer';
+import 'svelte/internal/disclose-version';
+import * as $ from 'svelte/internal/client';
+
+export default function Component($$anchor, $$props) {
+	var $$pop_renderer = $.push_renderer($renderer);
+	var fragment = $.comment();
+	var node = $.first_child(fragment);
+
+	$.snippet(node, () => $.validate_snippet_renderer($renderer, $$props.children) ?? $.noop);
+	$.append($$anchor, fragment);
+	$$pop_renderer();
+}

packages/svelte/tests/snapshot/samples/custom-renderer-single-node/_expected/client/main.svelte.js

@@ -0,0 +1,15 @@
+import $renderer from 'my-custom-renderer';
+import 'svelte/internal/disclose-version';
+import 'svelte/internal/flags/legacy';
+import * as $ from 'svelte/internal/client';
+import Component from "./Component.svelte";
+
+export default function Main($$anchor) {
+	var $$pop_renderer = $.push_renderer($renderer);
+	var fragment = $.comment();
+	var node = $.first_child(fragment);
+
+	Component(node, {});
+	$.append($$anchor, fragment);
+	$$pop_renderer();
+}

packages/svelte/tests/snapshot/samples/custom-renderer-single-node/_expected/server/Component.svelte.js

@@ -0,0 +1 @@
+export default function Component() {}

packages/svelte/tests/snapshot/samples/custom-renderer-single-node/_expected/server/main.svelte.js

@@ -0,0 +1 @@
+export default function Main() {}

packages/svelte/tests/snapshot/samples/custom-renderer-single-node/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	import Component from "./Component.svelte";
+</script>
+
+<Component />