How to give bottom border radius only in flexiblespace of SliverAppBar in flutter?

Question

I want to have a sliverAppBar with a rounded bottom border like this.

But it shouldn't be rounded when collapsed. Like this.

I tried a lot of solutions, but couldn't find anything.

SliverAppBar(
      leading: BackButton(color: Colors.black),
      pinned: true,
      snap: false,
      floating: false,
      expandedHeight: 250,
      backgroundColor: Color(0xffEBEBEB),
      flexibleSpace: FlexibleSpaceBar(
          centerTitle: true,
          title: Text("Collapsing Toolbar",
              style: TextStyle(
                color: Colors.red,               
                fontSize: 16.0,
              )),
         
          background: FlutterLogo()),
    ),
   
    SliverList(...)

Thanks in advance.