Reproduces DropdownButton menu location being off

main.dart

import 'package:flutter/material.dart';

class CustomMaterialPageRoute<T> extends MaterialPageRoute<T> {
  CustomMaterialPageRoute({
    @required WidgetBuilder builder,
    RouteSettings settings,
    bool maintainState = true,
    bool fullscreenDialog = false,
  }) : super(
    builder: builder,
    maintainState: maintainState,
    settings: settings,
    fullscreenDialog: fullscreenDialog,
  );

  @override
  Widget buildTransitions(
    BuildContext context,
    Animation<double> animation,
    Animation<double> secondaryAnimation,
    Widget child
    ) => child;
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Material(
        child: Center( // <- This makes the difference, i.e. change to a Scaffold, no issue.
          child: MyStatefulWidget()
        ),
      ),
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key}) : super(key: key);

  @override
  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  String _value;

  Widget build(BuildContext context) {
    return ConstrainedBox(
      // Changing the constraints here results in the dropdown menu showing up in different places
      constraints: new BoxConstraints.tight(const Size(300.0, 600.0)),
      child: Navigator(onGenerateRoute: (RouteSettings s) {
        return CustomMaterialPageRoute<void>(builder: (BuildContext context) {
          return Center(child:DropdownButton<String>(
            hint: Text('Select'),
            value: _value,
            onChanged: (String newValue) {
              setState(() {
                _value = newValue;
              });
            },
            items: <String>['One', 'Two', 'Three']
              .map<DropdownMenuItem<String>>((String value) {
              return DropdownMenuItem<String>(
                value: value,
                child: Text(value),
              );
            }).toList(),
          ),);
        });
      }),
    );
  }
}