WebviewScaffold alway on top

There is no way to display a FloatingActionButton over the webview. The button shows one sec then disapper. Stack seems not working. The WebviewScaffold remains always on top

Scaffold(
body: SafeArea(child: WebviewScaffold(
url: "http://192.168.4.100:8080",
withJavascript: true,
javascriptChannels: [
_alertJavascriptChannel(context),
].toSet(),
),),
floatingActionButton: Row(
mainAxisAlignment: MainAxisAlignment.end,
children: [
Visibility(
visible: _isFlashButtonOn,
child: FloatingActionButton(
onPressed: _toggleFlash,
tooltip: 'Toggle Flash',
backgroundColor: Colors.purple,
child: Icon(Icons.lightbulb_outline),
),),
SizedBox(
width: 10,
),
Visibility(
visible: _isButtonOn,
child: FloatingActionButton(
onPressed: _readBarcode,
tooltip: 'Scan Barcode',
child: Icon(Icons.camera_alt),
),),
],
),
),