如何在Flutter应用程序的顶部覆盖一个带有命名路由的小部件？

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Overlay Widget',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: HomePage(),
      routes: {
        '/overlay': (context) => OverlayPage(),
      },
    );
  }
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home'),
      ),
      body: Center(
        child: ElevatedButton(
          child: Text('Open Overlay'),
          onPressed: () {
            Navigator.pushNamed(context, '/overlay');
          },
        ),
      ),
    );
  }
}

class OverlayPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Overlay'),
      ),
      body: Stack(
        children: [
          // 底层内容
          Center(
            child: Text('This is the main content'),
          ),
          // 顶部覆盖的小部件
          Positioned(
            top: 0,
            left: 0,
            right: 0,
            child: Container(
              color: Colors.white,
              padding: EdgeInsets.all(16),
              child: Text(
                'This is the overlay widget',
                style: TextStyle(fontSize: 18),
              ),
            ),
          ),
        ],
      ),
    );
  }
}