fix(nested): add nested nodes example, always show nodes on top while dragging or selected

example/src/index.tsx

@@ -8,12 +8,13 @@ import Stress from './Stress';
 import CustomNode from './CustomNode';
 import FloatingEdges from './FloatingEdges';
 import Layouting from './Layouting';
+import NestedNodes from './NestedNodes';
 
 import './index.css';
 
 const routes = [
   {
-    path: '/basic',
+    path: '/',
     component: Basic,
   },
   {
@@ -36,6 +37,10 @@ const routes = [
     path: '/layouting',
     component: Layouting,
   },
+  {
+    path: '/nested-nodes',
+    component: NestedNodes,
+  },
 ];
 
 const Header = withRouter(({ history, location }) => {