import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  bool overflowing = true;

  _handleTap() {
    setState(() {
      overflowing = !overflowing;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Sandbox',
      home: Scaffold(
        body: CustomScrollView(
          shrinkWrap: true,
          slivers: <Widget>[
            /// AppBar
            SliverAppBar(
              pinned: true,
              actions: <Widget>[
                IconButton(
                  icon: Icon(Icons.swap_calls),
                  onPressed: _handleTap,
                ),
              ],
            ),

            /// Header
            SliverToBoxAdapter(
              child: Container(
                constraints: BoxConstraints(minHeight: overflowing ? 900 : 300),
                color: Colors.green.shade100,
                alignment: Alignment.center,
                child: Text("Header"),
              ),
            ),

            /// Footer
            SliverToBoxAdapter(
              child: Container(
                height: 100,
                color: Colors.blue.shade100,
                alignment: Alignment.center,
                child: Text("Footer"),
              ),
            ),
          ],
        ),
      ),
    );
  }
}