Repository: flutter
Issue: Multiple pinned widgets will cause the overlapsContent in the delegate of the SilverrPersistentHeader to return true.
Labels: framework,f: material design,f: scrolling,has reproducible steps,P2,found in release: 3.3,found in release: 3.6,team-design,triaged-design

Multiple pinned widgets(the `pinned` are set to `true`) will cause the `overlapsContent` in the `delegate` of the `SilverrPersistentHeader` to return `true`, as shown in the interface:

When sliding the `SliverFixedExtentList` under the `SliverPersistentHeader`, before the height of the `SliverPersistentHeader` starts to get smaller, a part of the `SliverFixedExtentList`(about 80 pixels) will be retracted to the bottom of the `SliverPersistentHeader` .

![UI](https://user-images.githubusercontent.com/17899073/63763785-02724a80-c8f8-11e9-941f-a724ff13bdf2.gif)

## Steps to Reproduce

1. 
```dart
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            pinned: true,
            expandedHeight: 180,
            flexibleSpace: FlexibleSpaceBar(
              collapseMode: CollapseMode.parallax,
              background: Container(
                color: Colors.lightBlue,
                child: Center(child: Text("title")),
              ),
            ),
          ),
          SliverToBoxAdapter(
            child: Text("divider"),
          ),
          SliverPersistentHeader(
            delegate: _SliverHeaderDelegate(200, 50),
            floating: false,
            pinned: true,
          ),
          SliverFixedExtentList(
            itemExtent: 50.0,
            delegate: SliverChildBuilderDelegate((BuildContext context, int index) {
              return Text('list item $index');
            }, childCount: 10),
          ),
        ],
        physics: BouncingScrollPhysics(),
        controller: _scrollController,
      ),
    );
  }
```
2. 
```dart
class _SliverHeaderDelegate extends SliverPersistentHeaderDelegate {
  double _maxExtent, _minExtent;

  _SliverHeaderDelegate(this._maxExtent, this._minExtent);

  @override
  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
    return SizedBox.expand(
      child: Container(
        color: overlapsContent ? Colors.red : Colors.lightBlue,
        child: Center(child: Text("$shrinkOffset")),
      ),
    );
  }

  @override
  double get maxExtent => _maxExtent;

  @override
  double get minExtent => _minExtent;

  @override
  bool shouldRebuild(SliverPersistentHeaderDelegate oldDelegate) {
    return oldDelegate != this || oldDelegate?.maxExtent != this.maxExtent || oldDelegate?.minExtent != this.minExtent;
  }
}
```



```
flutter analyze: No issues found! (ran in 6.6s)
```

```
[✓] Flutter (Channel beta, v1.8.3, on Mac OS X 10.14.6 18G87, locale zh-Hans-CN)
    • Flutter version 1.8.3 at /Users/YouCii/Library/flutter
    • Framework revision e4ebcdf6f4 (4 weeks ago), 2019-07-27 11:48:24 -0700
    • Engine revision 38ac5f30a7
    • Dart version 2.5.0 (build 2.5.0-dev.1.0 0ca1582afd)
 
[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
    • Android SDK at /Users/YouCii/Library/Android/sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-28, build-tools 28.0.3
    • ANDROID_HOME = /Users/YouCii/Library/Android/sdk
    • 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-1343-b01)
    • All Android licenses accepted.

[✓] Android Studio (version 3.4)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 38.2.1
    • Dart plugin version 183.6270
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)

[✓] Connected device (1 available)
    • MI 6 • 69841c30 • android-arm64 • Android 8.0.0 (API 26)

```