Flutter - using ListView.builder inside FutureBuilder and SliverList - scroll behaviour - flutter

Question

I'm trying to improve my app and want to wrap an existing FutureBuilder with a SliverList. Unfortunately I'm having a strange and incomprehensible scrolling behaviour.
Here's my code so far:
#override
Widget build(BuildContext context) {
return Container(
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
Expanded(
child: CustomScrollView(
slivers: [
SliverAppBar(
expandedHeight: 300,
floating: true,
flexibleSpace: FlexibleSpaceBar(
title: Padding(
padding: EdgeInsets.fromLTRB(30, 10, 30, 10),
child: Text(
'some subtext here...',
style: TextStyle(
fontStyle: FontStyle.italic,
color: Colors.white,
fontSize: 12.0)),
),
background: Stack(
fit: StackFit.expand,
children: [
Container(
child: Image.network(
'https://dummyimage.com/600x400/000/fff',
fit: BoxFit.cover,
),
),
const DecoratedBox(
decoration: BoxDecoration(
gradient: LinearGradient(
begin: Alignment(0.0, 0.5),
end: Alignment(0.0, 0.0),
colors: <Color>[
Color(0xcc000000),
Color(0x55000000),
],
),
),
),
],
),
),
),
SliverPadding(
padding: EdgeInsets.only(top: 10),
sliver: SliverList(
delegate: SliverChildListDelegate([
Container(
child: FutureBuilder(
future: fetchData(),
builder: (BuildContext context, AsyncSnapshot snapshot) {
if (snapshot.data == null) {
return Text('loading...');
} else {
return ListView.builder(
scrollDirection: Axis.vertical,
shrinkWrap: true,
itemCount: snapshot.data.length,
itemBuilder: (BuildContext content, int index) {
// return Text('Text ' + snapshot.data[index].name);
return snapshot.hasData ? Card(
elevation: 4,
child: ListTile(
leading: CircleAvatar(
backgroundImage:
NetworkImage(snapshot.data[index].image_url),
),
trailing: Text('Test'),
title: Text(snapshot.data[index].name),
subtitle: Text(snapshot.data[index].latin_name, style: TextStyle(
fontStyle: FontStyle.italic,
)),
onTap: () {
/*Navigator.push(context,
new MaterialPageRoute(builder: (context)=> DetailPage(snapshot.data[index]))
);*/
print('tapped');
},
),
) : Text('no Data');
},
);
//return Text('loading complete');
}
}
),
),
],),
),
)
],
)),
],
));
}
The SliverAppBar gets displayed correctly, but unfortunately since adding the Slivers my FutureBuilder doesn't display anything at all.
After adding scrollDirection and shrinkWrap to the ListView the data get's displayed but I'm unable to scroll correctly.
When dragging it at the red area the List snaps back up right after releasing The SliverAppBar doesn't shrink either. When scrolling in the green area the Sliver Header works correctly and the Scrolling doesn't snap right at the top when releasing.
Has anyone had this issue before? Or could someone explain on why this could happen?