main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      builder: (_, widget) {
        return Navigator(
          onGenerateRoute: (_) => MaterialPageRoute(builder: (_) => widget),
        );
      },
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      routes: {'/': (_) => MyHomePage()},
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final _key = GlobalKey();

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_) {
      final RenderBox _red = _key.currentContext.findRenderObject() as RenderBox;
      final offset = _red.localToGlobal(Offset.zero);
      Overlay.of(context).insert(OverlayEntry(builder: (_) {
        return Positioned(
          top: offset.dy,
          child: Container(
            color: Colors.blue,
            width: 50,
            height: 50,
          ),
        );
      }));
    });
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: GestureDetector(
        onTap: () => Navigator.of(context).pushNamed('/'),
        child: Container(
          key: _key,
          color: Colors.red,
          width: 50,
          height: 50,
        ),
      ),
    );
  }
}