When I scroll the list, the tab bar "vibrate" - flutter

Question

I have a TabBar in the AppBar and a ListView in the TabBarView. When I scroll the list, the TabBar "vibrate". If you notice at the gif below, there actually appears a little white space at the bottom of AppBar, below the tabs. This space appears and disappears so fast that it gives an impression of vibration. I want the TabBar to be at the bottom of the AppBar with no space below the TabBar.
The code of AppBar:
_appBar(){
return PreferredSize(
preferredSize: Size.fromHeight(160),
child: AppBar(
toolbarHeight: 70,
backgroundColor: Colors.white,
shadowColor: Colors.black,
elevation: 7,
centerTitle: true,
bottom: PreferredSize(
preferredSize: Size.fromHeight(90.0),
child: Container(
padding: EdgeInsets.only(left: 10, right: 10),
child: Column(
children: [
Container(
height: 40,
width: MediaQuery.of(context).size.width - 80,
decoration: BoxDecoration(
borderRadius: BorderRadius.circular(10),
color: Color.fromRGBO(243, 243, 243, 1),
),
child: Center(
child: Form(
child: TextFormField(
controller: searchController,
autocorrect: false,
keyboardType: TextInputType.visiblePassword,
textInputAction: TextInputAction.done,
decoration: InputDecoration(
contentPadding: EdgeInsets.only(right: 1, left: 14, top: 14, bottom: 14),
label: Text("Nome, nº de série"),
labelStyle: TextStyle(
fontFamily: "OpenSans",
fontSize: 16,
color: Color.fromRGBO(60, 60, 67, 0.6)
),
prefixIcon: Icon(Icons.search),
border: InputBorder.none,
floatingLabelBehavior: FloatingLabelBehavior.never,
),
),
),
),
),
SizedBox(height: 15),
TabBar(
controller: _tabController,
labelPadding: EdgeInsets.only(left: 5, right: 5),
indicatorColor: Theme.of(context).primaryColor,
indicatorSize: TabBarIndicatorSize.label,
indicatorWeight: 3.0,
labelColor: Colors.black,
labelStyle: TextStyle(
fontSize: 16,
fontFamily: "OpenSans"
),
tabs: const [
Tab(text: "RASCUNHOS"),
Tab(text: "EM PROCESSO"),
Tab(text: "FINALIZADO"),
]
)
],
),
),
),
),
);
}
I already removed the TabBar indicator decorations and the problem still happens.