An example of using iOS 13's draggable scrollbar with a CustomScrollView

draggable_scrollbar_custom_scroll_view.dart

import 'package:flutter/cupertino.dart';

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

class MyApp extends StatelessWidget {
  static List<Widget> _getList(int length) {
    final List<Widget> list = List<Widget>(length);
    for (int i = 0; i < length; i++) {
      list[i] = _getListItem(i);
    }
    return list;
  }

  static Widget _getListItem(int i) {
    return Text('list item $i');
  }

  @override
  Widget build(BuildContext context) {
    final ScrollController scrollController = ScrollController();
    return CupertinoApp(
      title: 'Flutter Demo',
      home: CupertinoPageScaffold(
        child: Column(
          children: <Widget>[
            Container(
              height: 600,
              child: PrimaryScrollController(
                controller: scrollController,
                child: CupertinoScrollbar(
                  controller: scrollController,
                  child: CustomScrollView(
                    slivers: <Widget>[
                      SliverFixedExtentList(
                        itemExtent: 50.0,
                        delegate: SliverChildListDelegate(_getList(100)),
                      ),
                    ],
                  ),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}