Button over bottomsheet-like layout

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.grey,
      child: SizedBox.expand(
        child: Column(
          children: [
            Flexible(
              child: Center(child: Text('Above')),
              flex: 1,
            ),
            Flexible(
              child: Stack(
                clipBehavior: Clip.none,
                children: [
                  Container(
                    color: Colors.white,
                    child: Center(
                      child: Text(
                        'Below',
                        style: TextStyle(color: Colors.black),
                      ),
                    ),
                  ),
                  Positioned(
                    right: 20,
                    top: -20,
                    child: Container(
                      color: Colors.green,
                      height: 40,
                      width: 40,
                    ),
                  ),
                ],
              ),
              flex: 2,
            ),
          ],
        ),
      ),
    );
  }
}