Repository: flutter
Issue: CupertinoSliverNavigationBar largeTitle gets bigger when transition finishes
Labels: e: device-specific,platform-ios,framework,f: scrolling,f: cupertino,a: quality,P2,team-ios,triaged-ios

## Steps to Reproduce

1. Run code in _Code sample_ section (Is a modified version of getting started code from Flutters web page).
2. Transition to page and go back

**Expected results:** Large title should have a smooth transition between text without opacity and with opacity.

**Actual results:** At the end of the transition the large title pops into a text with bigger size, the transition is not smooth.

![Example](https://media.giphy.com/media/CvyNE9Cff1olA97FFK/giphy.gif)

As you can see the large title pops bigger at the end of transition.

**Flutter doctor**
```
[✓] Flutter (Channel stable, 1.20.4, on Mac OS X 10.15.6 19G2021, locale en-ES)
[!] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
    ✗ Android license status unknown.
      Try re-installing or updating your Android SDK Manager.
      See https://developer.android.com/studio/#downloads or visit
      https://flutter.dev/docs/get-started/install/macos#android-setup for detailed instructions.
[✓] Xcode - develop for iOS and macOS (Xcode 12.0.1)
[!] Android Studio (version 3.6)
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
[✓] IntelliJ IDEA Ultimate Edition (version 2020.1)
 
[✓] Connected device (2 available)   
```

<details>
  <summary>Code sample</summary>


```
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:english_words/english_words.dart';

void main() => runApp(MyApp());

// #docregion MyApp
class MyApp extends StatelessWidget {
  // #docregion build
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Startup Name Generator',
      theme: ThemeData(
        primaryColor: Colors.white,
      ),
      home: RandomWords(),
    );
  }
// #enddocregion build
}
// #enddocregion MyApp

// #docregion RWS-var
class _RandomWordsState extends State<RandomWords> {
  final _suggestions = <WordPair>[];
  final _saved = <WordPair>{};
  final _biggerFont = TextStyle(fontSize: 18.0);

  // #enddocregion RWS-var

  // #docregion _buildSuggestions
  Widget _buildSuggestions() {
    return ListView.builder(
        padding: EdgeInsets.all(16.0),
        itemBuilder: /*1*/ (context, i) {
          if (i.isOdd) return Divider();
          /*2*/

          final index = i ~/ 2; /*3*/
          if (index >= _suggestions.length) {
            _suggestions.addAll(generateWordPairs().take(10)); /*4*/
          }
          return _buildRow(_suggestions[index]);
        });
  }

  // #enddocregion _buildSuggestions

  // #docregion _buildRow
  Widget _buildRow(WordPair pair) {
    final alreadySaved = _saved.contains(pair);
    return ListTile(
      title: Text(
        pair.asPascalCase,
        style: _biggerFont,
      ),
      trailing: Icon(
        alreadySaved ? Icons.favorite : Icons.favorite_border,
        color: alreadySaved ? Colors.red : null,
      ),
      onTap: () {
        setState(() {
          if (alreadySaved) {
            _saved.remove(pair);
          } else {
            _saved.add(pair);
          }
        });
      },
    );
  }

  // #enddocregion _buildRow

  // #docregion RWS-build
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: CupertinoNavigationBar(
        middle: Text('Startup Name Generator'),
        trailing: CupertinoButton(child: Icon(Icons.list), onPressed: _pushSaved),
      ),
      body: _buildSuggestions(),
    );
  }

  // #enddocregion RWS-build

  void _pushSaved() {
    Navigator.of(context).push(
      CupertinoPageRoute(
        // NEW lines from here...
        builder: (BuildContext context) {
          return Scaffold(
            body: CustomScrollView(
              slivers: [
                CupertinoSliverNavigationBar(
                  largeTitle: Text('test'),
                ),
                SliverList(
                    delegate: SliverChildListDelegate([
                  Padding(
                    padding: const EdgeInsets.all(20),
                    child: Text('test'),
                  ),
                ]))
              ],
            ),
          );
        }, //...to here.
      ),
    );
  }
// #docregion RWS-var
}
// #enddocregion RWS-var

class RandomWords extends StatefulWidget {
  @override
  State<RandomWords> createState() => _RandomWordsState();
}
```
</details>