Scale bug

main.dart

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  bool _hide = false;
  FocusNode _focusNode;

  @override
  void initState() {
    super.initState();
    _focusNode = new FocusNode();
    _focusNode.addListener(_handleFocusChange);
  }

  void _handleFocusChange() {
    setState(() {
      _hide = true;
    });
  }

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      resizeToAvoidBottomPadding: false,
      appBar: new AppBar(title: new Text(widget.title)),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new TextField(focusNode: _focusNode),
            new Text(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: _hide
          ? null
          : new FloatingActionButton(
              onPressed: _incrementCounter,
              tooltip: 'Increment',
              child: new Icon(Icons.add),
            ),
    );
  }
}