Repository: flutter
Issue: CupertinoTabScaffold prevents Textfield from gaining focus
Labels: a: text input,platform-android,framework,f: material design,f: cupertino,has reproducible steps,P3,found in release: 3.3,found in release: 3.6,team-design,triaged-design

I want to put TextField on AppBar. When i tap on TextField keyboard just appear for a second and then it disapear without any debug or error message.

<details>
  <summary>sample code</summary>


```dart
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:flutter_icons/flutter_icons.dart';
import 'package:flutter/cupertino.dart';
import 'dart:math';

class MainPage extends StatefulWidget {
  @override
  _MyMainPageState createState() => _MyMainPageState();
}

class _MyMainPageState extends State<MainPage> with SingleTickerProviderStateMixin {
  final _scaffoldKey = GlobalKey<ScaffoldState>();
  GlobalKey globalKey = new GlobalKey(debugLabel: 'btm_app_bar');
  final _navigatorKey = GlobalKey<NavigatorState>();
  CupertinoTabController _tabController;
  TextEditingController _txtContrSearch;
  FocusNode _focusSearch;
  var rng = new Random();

  @override
  void initState() {
    super.initState();
    _tabController = new CupertinoTabController();
    _txtContrSearch=TextEditingController();
    _focusSearch = FocusNode();
  }

  @override
  void dispose() {
    _tabController.dispose();
    _txtContrSearch?.dispose();
    _focusSearch?.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return 
            Scaffold(
                   key: _scaffoldKey,
                   backgroundColor: Colors.grey[100],
                   resizeToAvoidBottomInset: false,
                   appBar:
                      AppBar(
                        titleSpacing: 0,
                        leading:
                        GestureDetector(
                         child:
                          Icon(Icons.menu, size: 30,
                               color: Colors.black,
                          ),
                          onTap: () {
                            SystemSound.play(SystemSoundType.click);
                            _scaffoldKey.currentState.openDrawer();
                          },
                        ),
                        title:
                         Padding(padding: EdgeInsets.only(right: 5),
                         child:
                            Row(mainAxisAlignment: MainAxisAlignment.spaceBetween,
                            children: [
                              Expanded(flex: 1,
                                child:
                                Container(alignment: Alignment.center,
                                margin: EdgeInsets.only(top: 6, bottom: 5),
                                padding: EdgeInsets.only(left: 5, right: 5),
                                decoration:
                                  BoxDecoration(shape: BoxShape.rectangle, borderRadius: BorderRadius.circular(8),
                                                color: Colors.white, border: Border.all(width: 1, color: Colors.grey[350])
                                ),
                                child:
                                CupertinoTextField(
                                  controller: _txtContrSearch,
                                  focusNode: _focusSearch,
                                  decoration: null,
                                  textInputAction: TextInputAction.done,
                                  onSubmitted: (x) {
                                  },
                                )),
                              ),
                              SizedBox(
                                width: 5,
                              ),
                              SizedBox(height: 40, width: 40,
                              child:
                              Stack(alignment: Alignment.center,
                              children: [
                                Icon(Icons.chat_bubble_outline, size: 30, color: Color(0xFF6985AB),),
                                Positioned(left: 30/4, top: 6,
                                  child:
                                  Container(alignment: Alignment.center, height: 13,
                                  width: 13,
                                  decoration: BoxDecoration(shape: BoxShape.circle, color: Colors.redAccent),
                                  child:
                                  Text(rng.nextInt(100).toString(), textScaleFactor: 1.0, softWrap: false, overflow: TextOverflow.visible,),
                                  ))
                              ])),
                              Icon(AntDesign.getIconData('pushpino'), size: 30, color: Colors.black),
                            ])
                        ),
                        toolbarOpacity: 1,
                   ),
                   //bottomSheet: _bottomSheet(),
                   drawer: ....drawer(),                   
                   body:
                    SafeArea(child:
                        CupertinoTabScaffold(
                          resizeToAvoidBottomInset: false,
                          tabBar: CupertinoTabBar(
                            key: globalKey,
                            backgroundColor: Colors.white,
                            activeColor: Colors.blue,
                            inactiveColor: Colors.grey,
                            currentIndex: 0,
                            items: ...getTabItems(),
                            onTap: (x) {
                              SystemSound.play(SystemSoundType.click);
                            },
                          ),
                          tabBuilder: (_context, _index) {
                            return Container();
                          }
                        )
                    ),
    );
  }
}
```

</details>

If i remove `CupertinoTabScaffold` keyboard appear and work as intended.

Tested on Android 7.

## Flutter doctor
```
> Doctor summary (to see all details, run flutter doctor -v):
> [√] Flutter (Channel stable, v1.9.1+hotfix.4, on Microsoft Windows [Version 10.0.14393], locale en-US)
> 
> [√] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
> [√] Android Studio (version 3.4)
> [√] IntelliJ IDEA Ultimate Edition (version 2017.2)
> [√] Connected device (1 available)
> 
> • No issues found!
```