Flutter fails to report keyboard size via `MediaQuery.of(context).size`

view_insets.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text('ViewInsets Demo'),
        ),
        body: InputsPage(),
      ),
    );
  }
}

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

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

class _InputsPageState extends State<InputsPage> {
  FocusNode usernameFocusNode;
  FocusNode passwordFocusNode;

  @override
  void initState() {
    super.initState();
    usernameFocusNode = FocusNode();
    passwordFocusNode = FocusNode();

    usernameFocusNode.addListener(() {
      print(MediaQuery.of(context).size);
      print(MediaQuery.of(context).viewInsets);
      print(MediaQuery.of(context).viewInsets.bottom);
    });
    passwordFocusNode.addListener(() {
      print(MediaQuery.of(context).size);
      print(MediaQuery.of(context).viewInsets);
      print(MediaQuery.of(context).viewInsets.bottom);
    });
  }

  @override
  void dispose() {
    usernameFocusNode.dispose();
    passwordFocusNode.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
          TextFormField(
            decoration: InputDecoration(
              hintText: 'Username',
            ),
            focusNode: usernameFocusNode,
          ),
          SizedBox(height: 100),
          TextFormField(
            decoration: InputDecoration(
              hintText: 'Password',
            ),
            focusNode: passwordFocusNode,
            obscureText: true,
          ),
        ],
      ),
    );
  }
}