Repository: flutter
Issue: PageView starts UNDER SliverAppBar in a NestedScrollView rather than BELOW it
Labels: framework,f: material design,f: scrolling,a: quality,has reproducible steps,P2,found in release: 3.3,found in release: 3.5,team-design,triaged-design

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Motivation: To create page that has a SliverAppBar and a PAGEView that is controlled by a horizontal SingleChildScrollView containing a row of children
As of now I am just focusing in putting the PAGEVIEW in correct alignment and this is where the issue arises.

## Steps to Reproduce
Attached is a image of desired final layout
<!-- You must include full steps to reproduce so that we can reproduce the problem. -->

1. Create a new Flutter project and replace the code in the main file with the code below.
2. Create a folder 'assets/images/' under the project and put an image file in it.
3. Put the name of the image file inline 24 under <flexibleSpace: FlexibleSpaceBar( background: Image.asset()>
4. Execute the project

**Expected results:** A NestedScrollView with SliverAppBar and a PageView BELOW it along with pageview text in it

**Actual results:** A NestedScrollView with SliverAppBar and a PageView UNDER it along with pageview text in it
The PAGEVIEW does not start BELOW or AFTER SliverAppBar, rather it is UNDER the SliverAppBar, as if they were stacked.
The text on each page has been given different padding to show the effect as the text with 0 padding does not appear because it is under SliverAppBar

<==================Code to run==================>
```
[import 'package:flutter/material.dart';

class EnglishPicturesCards extends StatelessWidget {
  static final controller = PageController();
  List<Widget> pages = [
    Container(color: Colors.red, child: Text('Hello'), padding: EdgeInsets.fromLTRB(0, 200, 0, 0),),
    Container( color: Colors.blue, child: Text('Mellow'),padding: EdgeInsets.fromLTRB(0, 150, 0, 0),),
    Container( color: Colors.green,  child: Text('Yellow'),),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        child: NestedScrollView(
          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
            return <Widget>[
              SliverOverlapAbsorber(
                handle:NestedScrollView.sliverOverlapAbsorberHandleFor(context),
                sliver: SliverAppBar(
                  leading: Builder(builder: (BuildContext context) { return IconButton( icon: Icon(Icons.home,color: Colors.white,), onPressed: null, );}),
                  pinned: true,
                  expandedHeight: 150.0,
                  flexibleSpace: FlexibleSpaceBar( background: Image.asset( 'assets/images/<ANY IMAGE>.jpg', fit: BoxFit.cover, ),),
                  title: Text( 'English Words', style: TextStyle(color: Colors.white, fontSize: 25), ),
                  backgroundColor: Colors.lightBlueAccent,
                ),
              ),
            ];
          },
          body: PageView(controller: controller, children: pages, ),
        ),
      ),
    );
  }
}](url)
``
<==================Code to run==================>

<details>
  <summary>Logs</summary>

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, v1.17.5, on Microsoft Windows [Version 10.0.18363.476], locale nb-NO)

[√] Android toolchain - develop for Android devices (Android SDK version 30.0.0)
[√] Android Studio (version 4.0)
[√] Connected device (1 available)

• No issues found!

```
![desiredPagelayout](https://user-images.githubusercontent.com/51916021/88466674-12f7c000-cecf-11ea-976c-0e7339890498.PNG)


</details>