Flutter SliverList Overlap on SliverAppBar

I'm trying to overlap a SliverList a few pixels over the SliverAppBar. I'd like the image in the FlexibleSpaceBar to go under the radius of my SliverList. I can only get the radius. Without the ability to overlap the SliverList onto the SliverAppBar.

return CustomScrollView(
      physics: BouncingScrollPhysics(),
      slivers: [
        SliverAppBar(
          backgroundColor: Colors.transparent,
          elevation: 0.0,
          expandedHeight: getProportionateScreenHeight(350),
          automaticallyImplyLeading: false,
          pinned: true,
          stretch: true,
          leading: Container(
            padding: EdgeInsets.fromLTRB(8.0, 8.0, 0.0, 0.0),
            child: SizedBox(
              height: getProportionateScreenWidth(40),
              width: getProportionateScreenWidth(40),
              child: FlatButton(
                shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.circular(60),
                ),
                color: Colors.white,
                padding: EdgeInsets.zero,
                child: Icon(Icons.arrow_back_ios),
                onPressed: () => Navigator.pop(context),
              ),
            ),
          ),
          actions: <Widget>[
            Container(
              padding: EdgeInsets.fromLTRB(0.0, 8.0, 8.0, 0.0),
              child: SizedBox(
                height: getProportionateScreenWidth(40),
                width: getProportionateScreenWidth(40),
                child: FlatButton(
                  shape: RoundedRectangleBorder(
                    borderRadius: BorderRadius.circular(60),
                  ),
                  color: Colors.white,
                  padding: EdgeInsets.zero,
                  child: Icon(Icons.more_vert),
                  onPressed: () {}
                ),
              ),
            ),
          ],
          flexibleSpace: FlexibleSpaceBar(
            //title: Text(_event.Title),
            centerTitle: true,
            stretchModes: [
              StretchMode.zoomBackground
            ],
            background: Stack(
              fit: StackFit.expand,
              children: <Widget>[
                Image.asset('assets/images/events/sunset.jpg', fit: BoxFit.cover),
                DecoratedBox(
                  decoration: BoxDecoration(
                    gradient: LinearGradient(
                      begin: Alignment(0.0, 0.5),
                      end: Alignment(0.0, 0.0),
                      colors: <Color>[
                        Color(0x60000000),
                        Color(0x00000000),
                      ],
                    ),
                  ),
                ),
              ]
            )
          )
        ),
        SliverList(
          delegate: SliverChildListDelegate([
            Column(
              children: [
                Container(
                  padding: EdgeInsets.only(top: getProportionateScreenHeight(20), bottom: getProportionateScreenHeight(100)),
                  child: EventDescription(event: _event),
                  decoration: BoxDecoration(
                    color: Color(0xFFF5F6F9),
                    borderRadius: BorderRadius.only(
                      topLeft: Radius.circular(40),
                      topRight: Radius.circular(40),
                    ),
                    boxShadow: [
                      BoxShadow(
                        color: Colors.grey.withOpacity(0.5),
                        spreadRadius: 5,
                        blurRadius: 7,
                        offset: Offset(0, 0), // changes position of shadow
                      ),
                    ],
                  )
                ),
              ],
            )
          ]),
        )
      ]
    );

Would you like to use a workaround solution? My solution is put the top part of the SliverList inside SliverAppBar.

The only problem is that the fade out animation of flexiable space (while scrolling down) do effect to the top bar. (Because it is in the flexibleSpace)

Due to SliverAppBar elevation is higher than SliverList. Any Scale or Translate to the widget always get the opposite result from your expected (SliverAppBar overlap on SliverList).

I use stack to put the top part inside FlexibleSpaceBar, set collapseMode to CollapseMode.pin and set alignment to Alignment.bottomCenter:

SliverAppBar(
  ...
  flexibleSpace: FlexibleSpaceBar(
    collapseMode: CollapseMode.pin,
    ...
    background: Stack(
      ...
      children: [
        ...
        Align(
          alignment: Alignment.bottomCenter,
          child: Container(
            child: Container(
              height: _height, // 40
            ),
            decoration: BoxDecoration(
              color: Color(0xFFF5F6F9),
              borderRadius: BorderRadius.only(
                topLeft: Radius.circular(40),
                topRight: Radius.circular(40),
              ),
              boxShadow: [
                BoxShadow(
                  color: Colors.grey.withOpacity(0.5),
                  spreadRadius: 5,
                  blurRadius: 7,
                  offset: Offset(0, 0), // changes position of shadow
                ),
              ],
            ),
          ),
        ),
      ]
     ...

If you need to use collapseMode: CollapseMode.parallax like the default setting do, the _height need to be dynamical during scrolling.

void initState(){
  _scrollController = ScrollController()..addListener(() {
    setState(() {
      _height = math.max(_scrollController.offset*_topBarFactor +_topBarHeight,_topBarHeight);
    });
}
});