import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: MyStatelessWidget(),
    );
  }
}

class MyStatelessWidget extends StatelessWidget {
  MyStatelessWidget({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final List<String> _tabs = ['Tab 1', 'Tab 2'];
    final paddingTop = MediaQuery.of(context).padding.top;
    return DefaultTabController(
      length: _tabs.length, // This is the number of tabs.
      child: Scaffold(
        body: NestedScrollView(
          floatHeaderSlivers: true,
          headerSliverBuilder: (context, innerBoxIsScrolled) => [

            // SliverAppBar(
            //   pinned: false,
            //   toolbarHeight: 0.0,
            //   collapsedHeight: 0.0,
            //   expandedHeight: null,
            //   primary: false,
            // ),

            //********* or *********

            // SliverOverlapAbsorber(
            //   handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
            //   sliver: SliverAppBar(
            //     pinned: true,
            //     toolbarHeight: 62.0,                    // 63.0 works fine
            //     collapsedHeight: 62.0,                  // 63.0 works fine
            //     expandedHeight: null,
            //     primary: false,
            //   ),
            // ),

            //********* or *********

            // SliverOverlapAbsorber(
            //   handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
            //   sliver: SliverAppBar(
            //     pinned: true,
            //     toolbarHeight: 62.0 - paddingTop,    // 63.0 - paddingTop works fine
            //     collapsedHeight: 62.0 - paddingTop,  // 63.0 - paddingTop works fine
            //     expandedHeight: null,
            //     primary: true,
            //   ),
            // ),

            //********* or *********

            // SliverOverlapAbsorber(
            //   handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
            //   sliver: SliverAppBar(
            //     pinned: true,
            //     toolbarHeight: kToolbarHeight,
            //     collapsedHeight: null,
            //     expandedHeight: 56.0,                // 57.0 works fine
            //     primary: false,
            //   ),
            // ),
          ],
          body: TabBarView(
            // These are the contents of the tab views, below the tabs.
            children: _tabs.map((String name) {
              return SafeArea(
                top: false,
                bottom: false,
                child: Builder(
                  builder: (BuildContext context) {
                    return CustomScrollView(
                      key: PageStorageKey<String>(name),
                      slivers: <Widget>[
                        SliverOverlapInjector(
                          handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
                        ),
                        SliverPadding(
                          padding: const EdgeInsets.all(8.0),
                          sliver: SliverFixedExtentList(
                            itemExtent: 48.0,
                            delegate: SliverChildBuilderDelegate(
                              (BuildContext context, int index) {
                                return ListTile(
                                  title: Text('Item $index'),
                                );
                              },
                              childCount: 30,
                            ),
                          ),
                        ),
                      ],
                    );
                  },
                ),
              );
            }).toList(),
          ),
        ),
      ),
    );
  }
}