如何减少抖动中下拉列表的高度

Question

如何减少Flutter中的下拉列表高度。现在它覆盖了全屏。

    import 'package:flutter/material.dart';

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

class MyApp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _MyAppState();
  }
}

class _MyAppState extends State<MyApp> {
  List<DropdownMenuItem<String>> _dropdownMenuItemsyears;
  String _selectedyear;

  onChangeDropdownItemyear(String selectedYear) {
    setState(() {
      _selectedyear = selectedYear;
    });
  }

  List<String> getYears() {
    List<String> years = new List<String>();
    var date = new DateTime(1900).year;
    var now = new DateTime.now().year;
    int dateFrom = date;
    int dateTo = now;
    for (int i = dateFrom; i <= dateTo; i++) {
      years.add(i.toString());
    }
    years.add("Select");
    return years.reversed.toList();
  }

  List<String> _yearvalue = new List<String>();

  List<DropdownMenuItem<String>> buildDropdownMenuItemsyears(List Years) {
    List<DropdownMenuItem<String>> itemsyears = List();
    for (String Year in Years) {
      itemsyears.add(
        DropdownMenuItem(
          value: Year,
          child: Text(
            Year,
            overflow: TextOverflow.ellipsis,
            style: new TextStyle(
                fontFamily: 'Muli',
                color: Colors.black,
                fontSize: 16,
                decoration: TextDecoration.none),
          ),
        ),
      );
    }
    return itemsyears;
  }

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    _yearvalue = getYears();
    _dropdownMenuItemsyears = buildDropdownMenuItemsyears(_yearvalue);
    _selectedyear = _dropdownMenuItemsyears[0].value;
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('First app')),
        body: Column(children: [
          Container(
              child: new Container(
            width: 150,
            transform: Matrix4.translationValues(-6, -12, 0),
            child: new Container(
              child: DropdownButtonHideUnderline(
                child: ButtonTheme(
                  alignedDropdown: true,
                  child: DropdownButton(
                    value: _selectedyear,
                    items: _dropdownMenuItemsyears,
                    //Text widgets that have more text and are larger than the hint
                    onChanged: onChangeDropdownItemyear,
                  ),
                ),
              ),
            ),
          )),
        ]),
      ),
    );
  }
}