Flutterスクロールの視覚差

new CustomScrollView(
    physics: new BouncingScrollPhysics(),
    //      slivers    
    slivers: [
        //     
        new SliverAppBar(
            //   
            expandedHeight: 256.0,
            pinned: true,
            floating: !true,
            snap: !true,
            flexibleSpace: new FlexibleSpaceBar(
                //   
                title: Text('  '),
                centerTitle: true,
                //    
                background: new Image.network(
                    'http://letsfilm.org/wp-content/uploads/2019/03/everaldo-coelho-486014-unsplash.jpg',
                    fit: BoxFit.cover,
                ),
            ),
        ),
        //     
        new SliverList(
            delegate: new SliverChildBuilderDelegate((ctx, index) {
                return new Text('item: $index');
            }, childCount: this.count),
        ),
    ],
)