Sunflower

index.html

<h2>Btn example</h2>

<div>
  <canvas id="canvas" width="300" height="300"></canvas>
</div>

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      home: new MyHomePage(),
    );
  }
}

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

class _MyHomePageState extends State<MyHomePage> {
  var expanded = true;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: [
          Stack(
            children: <Widget>[
              Row(
                children: [
                  Container(
                    color: Colors.green,
                    height: 120,
                    width: MediaQuery.of(context).size.width * 0.50,
                  ), //Sub btn1

                  Container(
                    color: Colors.red,
                    height: 120,
                    width: MediaQuery.of(context).size.width * 0.50,
                  ), //Sub btn2
                ],
              ),
              

              GestureDetector(
                child: AnimatedContainer(
                  duration: const Duration(milliseconds: 200),
                  height: expanded ? 120 : 0,
                  child: Container(
                    height: 120,
                    color: Colors.grey[300],
                  ),  
                ),
                onTap: () => setState(() {
                  expanded = !expanded;
                })
              ), //Main btn
            ]          
          ),
        ],
      ),
    );
  }
}

styles.css

/* Copyright 2011 the Dart project authors. All rights reserved. */
/* Use of this source code is governed by a BSD-style license */
/* that can be found in the LICENSE file. */

h2 {
  margin-bottom: 0;
  text-align: center;
}

div {
  text-align: center;
}