Repository: flutter
Issue: IconButton Ink effect is drawn underneath SliverAppBar (inside a NestedScrollView)
Labels: framework,f: material design,a: quality,has reproducible steps,P2,found in release: 3.0,found in release: 3.1,team-design,triaged-design

A `Stack` with two children - a `NestedScrollView` and an `IconButton` causes the `Ink` effect of the `IconButton` to be drawn under the `SliverAppBar` contained within the `NestedScrollView`.

### Steps to reproduce
1. Run the code below:
```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: 'IconButton Bug',
      home: IconButtonBugView(),
    );
  }
}

class IconButtonBugView extends StatefulWidget {
  @override
  _IconButtonBugViewState createState() => _IconButtonBugViewState();
}

class _IconButtonBugViewState extends State<IconButtonBugView> {
  @override
  Widget build(BuildContext context) {
    // setState(() {});

    return Scaffold(
      body: Stack(
        children: <Widget>[
          NestedScrollView(
            headerSliverBuilder:
                (BuildContext context, bool innerBoxIsScrolled) {
              return <Widget>[
                SliverAppBar(
                  flexibleSpace: FlexibleSpaceBar(
                    title: Padding(
                      padding: const EdgeInsets.only(top: 28.0),
                      child: Row(
                        mainAxisAlignment: MainAxisAlignment.spaceBetween,
                        children: <Widget>[
                          Text('Title'),
                          IconButton(
                            icon: Icon(Icons.chevron_right),
                            // Uncomment below to make splash color more pronounced.
                            // splashColor: Color(0xffFF0000),
                            tooltip: 'Click me',
                            onPressed: () {},
                          ),
                        ],
                      ),
                    ),
                  ),
                ),
              ];
            },
            body: Center(
              child: Text('Body'),
            ),
          ),
          SafeArea(
            child: IconButton(
              icon: Icon(Icons.chevron_left),
              // Uncomment below to make splash color more pronounced.
              // splashColor: Color(0xffFF0000),
              tooltip: 'Click me',
              onPressed: () {},
            ),
          ),
        ],
      ),
    );
  }
}
```
2. Click on the _Right Chevron_. Note the splash effect is properly drawn above the app bar.
3. Scroll the view so that the app bar is partially visible.
4. Click on the _Left Chevron_. Note the splash effect is incorrectly drawn underneath the app bar.

## Logs

```
Analyzing app...                                                        
No issues found! (ran in 7.1s)

[✓] Flutter (Channel stable, v1.2.1, on Mac OS X 10.14.3 18D109, locale en-IL)
    • Flutter version 1.2.1 at /Users/benPesso/Projects/flutter
    • Framework revision 8661d8aecd (3 weeks ago), 2019-02-14 19:19:53 -0800
    • Engine revision 3757390fa4
    • Dart version 2.1.2 (build 2.1.2-dev.0.0 0a7dcf17eb)

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
    • Android SDK at /Users/benPesso/Library/Android/sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-28, build-tools 28.0.3
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1248-b01)
    • All Android licenses accepted.

[✓] iOS toolchain - develop for iOS devices (Xcode 10.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 10.1, Build version 10B61
    • ios-deploy 1.9.4
    • CocoaPods version 1.5.3

[✓] Android Studio (version 3.3)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 33.3.1
    • Dart plugin version 182.5215
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1248-b01)

[✓] VS Code (version 1.31.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 2.24.0

[✓] Connected device (1 available)
    • Android SDK built for x86 • emulator-5554 • android-x86 • Android 8.1.0 (API 27) (emulator)

• No issues found!
```