Disable with stack widget.

I just set widget like this.

Scaffold(
body: Stack(
children: [
Align(
alignment: Alignment.center,
child: SafeArea(
left: Platform.isAndroid ?? true,
top: Platform.isAndroid ?? true,
right: Platform.isAndroid ?? true,
bottom: Platform.isAndroid ?? true,
child: WebviewScaffold(
key: _shopViewKey,
primary: false,
hidden: true,
withOverviewMode: true,
initialChild: Container(
width:size.width*1,
height:size.height*1,
color:Colors.white,
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
Image.asset(
'assets/p_red_logo.png',
fit: BoxFit.cover,
width: mw(190),
height:mw(50),
),
Padding(
padding: EdgeInsets.only(top: mw(19), bottom: mw(20)),
child: SizedBox(
width: mw(26),
height: mw(26),
child: CircularProgressIndicator(
valueColor: new AlwaysStoppedAnimation(Color(0XFFec3e39)),
),
),
),
Padding(
padding: EdgeInsets.only(bottom:mw(20)),
child: Text('쇼핑몰로 이동중이에요!', style: TextStyle(fontSize: ScreenUtil().setSp(16), fontWeight: FontWeight.w900),),
),
Text('피클링에서 제공하는 상품정보는', style: TextStyle(fontSize: ScreenUtil().setSp(12), fontWeight: FontWeight.w500, color: Color(0XFF8a8a8a)),),
Text('쇼핑몰과 일치하지 않을 수 있습니다.', style: TextStyle(fontSize: ScreenUtil().setSp(12), fontWeight: FontWeight.w500, color: Color(0XFF8a8a8a)),),
Text('방문한 쇼핑몰에서 반드시 상품정보를 확인해주시기 바랍니다.', style: TextStyle(fontSize: ScreenUtil().setSp(12), fontWeight: FontWeight.w500, color: Color(0XFF8a8a8a)),),

Padding(
padding: EdgeInsets.only(top: mw(10)),
child: Text('(주)마들렌메모리는 통신판매의 당사자가 아니며,', style: TextStyle(fontSize: ScreenUtil().setSp(12), fontWeight: FontWeight.w500, color: Color(0XFF8a8a8a)),),
),
Text('상품에 대한 주문 및 배송, 환불 등에 대한 의무와 책임은', style: TextStyle(fontSize: ScreenUtil().setSp(12), fontWeight: FontWeight.w500, color: Color(0XFF8a8a8a)),),
Text('해당 쇼핑몰에 있습니다.', style: TextStyle(fontSize: ScreenUtil().setSp(12), fontWeight: FontWeight.w500, color: Color(0XFF8a8a8a)),),

],
),
),
supportMultipleWindows: true,
withJavascript: true,
debuggingEnabled: true,
resizeToAvoidBottomInset: true,
useWideViewPort:true,
url: Uri.encodeFull(widget.viewData[0]),
),
),
),
_visible == true && _isMarked == true ?
Positioned.fill(
child: FadeTransition(
opacity: Tween(begin: 0.0, end: 1.0).animate(animationController),
child: ScaleTransition(
scale: Tween(begin: 0.5, end: 1.5,).animate(CurvedAnimation(
parent: animationController,
curve: Curves.elasticOut,
reverseCurve: Curves.fastOutSlowIn
)),
child :Icon(Icons.favorite, size: mw(100), color:Color(0XFFed3d47),)
),
)
): Container(),
Positioned(
top:mw(20),
left:mw(20),
child: Container(color:Colors.blue, width: mw(100), height:mw(100)),
)
],
),
bottomNavigationBar: BottomNavigationBar(
currentIndex: _bottomSelectedIndex,
selectedItemColor: Colors.black,
unselectedItemColor: Colors.black,
showUnselectedLabels: true,
type: BottomNavigationBarType.fixed,
onTap: (index) {
bottomTapped(index, $user, $item, context);
},
items: buildBottomNavBarItems(),
),
);

So, I think. blue color container always on top in Widget tree.
But, blue container is not seen.
Also, it seems very weird, when I use flutter toast, the toast is always top in the widget tree and it visible.

So, How can it be?