SliverAppBar with title and content and when scrolled content is not displayed

appbar.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MainCollapsingToolbar(),
    );
  }
}

class MainCollapsingToolbar extends StatefulWidget {
  @override
  _MainCollapsingToolbarState createState() => _MainCollapsingToolbarState();
}

class _MainCollapsingToolbarState extends State<MainCollapsingToolbar> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: DefaultTabController(
        length: 2,
        child: NestedScrollView(
          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
            return <Widget>[
              SliverAppBar(
                expandedHeight: 200.0,
                floating: false,
                pinned: true,
                flexibleSpace: FlexibleSpaceBar(
                  centerTitle: true,
                  title: const Text(
                    "Collapsing Toolbar",
                    style: TextStyle(
                      color: Colors.white,
                      fontSize: 16.0,
                    ),
                  ),
                  background: Image.network(
                    "https://images.pexels.com/photos/396547/pexels-photo-396547.jpeg?auto=compress&cs=tinysrgb&h=350",
                    fit: BoxFit.cover,
                  ),
                ),
              ),
              SliverPersistentHeader(
                floating: true,
                
                delegate: _SliverAppBarDelegate(
                  const TabBar(
                    labelColor: Colors.black87,
                    unselectedLabelColor: Colors.grey,
                    tabs: [
                      Tab(
                        icon: Icon(Icons.info),
                        text: "Tab 1",
                      ),
                      Tab(
                        icon: Icon(Icons.lightbulb_outline),
                        text: "Tab 2",
                      ),
                    ],
                  ),
                ),
              ),
            ];
          },
          body: const Center(
            child: Text("Sample text"),
          ),
        ),
      ),
    );
  }
}

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 Container(
      child: _tabBar,
    );
  }

  @override
  bool shouldRebuild(_SliverAppBarDelegate oldDelegate) {
    return false;
  }
}