title: Why there is a shadow between nested navigator?
body: <p>When i use navigator nested in MaterialApp that contains a navigator, there is shadow as showed in the following image.</p>
  
  <p>Why? How to remove this shadow?</p>
  
  <p><a href="https://i.stack.imgur.com/1kWcP.png" rel="noreferrer"><img src="https://i.stack.imgur.com/1kWcP.png" alt="enter image description here"></a></p>
  
  <p>My code is here:</p>
  
  <pre><code>import 'package:flutter/material.dart';
  
  void main() =&gt; runApp(NavigatorNested());
  
  class NavigatorNested extends StatefulWidget {
      @override
      NavigatorNestedState createState() =&gt; NavigatorNestedState();
  }
  
  class NavigatorNestedState extends State&lt;NavigatorNested&gt; {
      @override
      Widget build(BuildContext context) {
          return MaterialApp(
              initialRoute: 'hpme',
              title: 'test',
              theme: ThemeData(
                  primarySwatch: Colors.blue,
              ),
              home: Scaffold(
                  appBar: AppBar(
                      title: Text('helle world'),
                  ),
                  body: Container(
                          margin: EdgeInsets.symmetric(vertical: 48, horizontal: 24),
                          child: Navigator(
                          onGenerateRoute: (settings) {
                              return MaterialPageRoute&lt;dynamic&gt;(
                                  settings: settings,
                                  builder: (context) {
                                  return Text('helle world');
                              });
                          },
                          onUnknownRoute: (settings) {
                              return MaterialPageRoute&lt;dynamic&gt;(
                                  settings: settings,
                                  builder: (context) {
                                  return Text('helle world');
                              });
                          },
                          initialRoute: "home",
                      ),
                  ),
              ),
          );
      }
  }
  </code></pre>
  
  <p>If i use <code>Text('hello')</code> to replace <code>Navigator</code>, it works goods. Is it a bug in flutrer?</p>