Repository: flutter
Issue: Suggestion: CupertinoSliverNavigationBar allow forcing condensed title
Labels: framework,a: fidelity,f: cupertino,c: proposal,P3,team-design,triaged-design

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## Use case

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See: iOS 13 Settings app search, App Store Search, or News app's Following tab search.

While searching and viewing results, the title is hidden.

My thoughts were something like this:
```dart
  Widget _buildSliverBar(BuildContext context) {
    return CupertinoSliverNavigationBar(
      middle: _isSearching
          ? TextField(
              autofocus: true,
              controller: _filter,
              decoration: InputDecoration(
                  hintText: 'Search by name...', border: InputBorder.none),
            )
          : null,
      largeTitle: _isSearching ? null : _buildTitle(),
      trailing: IconButton(
        icon: Icon(_isSearching ? Icons.clear : Icons.search),
        onPressed: () {
          setState(() {
            _isSearching = !_isSearching;
            if (!_isSearching) {
              _resetTitle();
            }
          });
        },
      ),
    );
  }
```

Alternatives: The built-in iOS apps that I mentioned probably actually behave by hiding the title bar and having a separate pinned search bar, e.g. `SliverPersistentHeader`. `SliverPersistentHeader`, however, doesn't seem to be very easily extensible.

`SliverAppBar` has a `bottom` option for easily appending a widget as well as allowing `flexibleSpace` (analogous to `largeTitle`) to be set to `null`, while placing widgets in `title` (analogous to `middle`).

## Proposal

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### Proposal:
![Simulator Screen Shot - iPhone SE (2nd generation) - 2020-06-15 at 23 33 49](https://user-images.githubusercontent.com/6323077/84729177-0cd20380-af61-11ea-8270-2e22a0b337a0.png)

### Apple News app
![IMG_0512](https://user-images.githubusercontent.com/6323077/84728986-959c6f80-af60-11ea-976c-a5aa6fda0ef3.jpg)
![IMG_0513](https://user-images.githubusercontent.com/6323077/84728984-959c6f80-af60-11ea-8b31-c46ceb21d007.jpg)
![IMG_0514](https://user-images.githubusercontent.com/6323077/84728980-9503d900-af60-11ea-9e2c-4e711cf50ce7.jpg)