当我开始向上滚动时，我的sliverappbar不会展开，只有当我再次到达列表顶部时它才会展开

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: CustomScrollView(
          controller: ScrollController(),
          slivers: <Widget>[
            SliverAppBar(
              expandedHeight: 200,
              flexibleSpace: FlexibleSpaceBar(
                title: Text('SliverAppBar Demo'),
                background: Image.network(
                  'https://example.com/image.jpg',
                  fit: BoxFit.cover,
                ),
              ),
            ),
            SliverList(
              delegate: SliverChildBuilderDelegate(
                (BuildContext context, int index) {
                  return ListTile(
                    title: Text('Item $index'),
                  );
                },
                childCount: 100,
              ),
            ),
          ],
        ),
      ),
    );
  }
}