Tab bar content is scrolling up until the status bar while creating collapsing tool bar with tabs - flutter

Question

I am trying to implement a collapsing toolbar with tabs. While scrolling through the content of the tab are not sticking to the bottom of the tab, rather it's scrolling up until the status bar.
I am not sure what exactly I am doing wrong. I'd like to get your help to solve this issue.
Here is the code snippet:
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:mobile/src/data/model/order_response.dart';
import 'package:mobile/src/widget/order_view.dart';
class OrderDetailPage extends StatefulWidget {
final OrderResponse item;
OrderDetailPage(this.item);
#override
_OrderDetailPageState createState() => _OrderDetailPageState();
}
class _OrderDetailPageState extends State<OrderDetailPage> {
#override
Widget build(BuildContext context) {
return Scaffold(
body: DefaultTabController(
length: 2,
child: NestedScrollView(
headerSliverBuilder:
(BuildContext context, bool innerBoxIsScrolled) {
return <Widget>[
SliverAppBar(
backgroundColor: Colors.black12,
elevation: 0.0,
expandedHeight: 230,
floating: false,
pinned: true,
flexibleSpace: FlexibleSpaceBar(
collapseMode: CollapseMode.parallax,
background: Align(
alignment: Alignment.bottomCenter,
child: Container(
child: Column(
children: [
SizedBox(
height: 90,
),
OrderListItemView(widget.item, null)
],
),
),
),
),
),
SliverPersistentHeader(
pinned: true,
delegate: _SliverAppBarDelegate(
TabBar(
labelColor: Colors.black87,
unselectedLabelColor: Colors.grey,
tabs: [
new Tab(text: "Instant Match"),
new Tab(text: "Requests"),
],
),
),
)
];
},
body: new TabBarView(
children: <Widget>[
Text(
'This content should stick to the bottom of tab bar',
style: TextStyle(fontSize: 24),
),
Text('This content should stick to the bottom of tab bar')
],
)),
),
);
}
void _onClickMenu(String value) {}
}
class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
_SliverAppBarDelegate(this._tabBar);
final TabBar _tabBar;
#override
double get minExtent => _tabBar.preferredSize.height;
#override
double get maxExtent => _tabBar.preferredSize.height;
#override
Widget build(
BuildContext context, double shrinkOffset, bool overlapsContent) {
return new Container(
child: _tabBar,
);
}
#override
bool shouldRebuild(_SliverAppBarDelegate oldDelegate) {
return false;
}
}
Current output:
In the desired output the content should stick to the bottom of tab bar.