Flashing page when select combobox

[SteMMo]

Hi all,
I'm developing a kiosk machine using QML and WebEngine module.
Environment: Win10 + Qt 5.9.9 +
import QtQuick 2.7
import QtQuick.Window 2.2
import QtQuick.VirtualKeyboard 2.2
import QtWebEngine 1.1

Everything is OK but when I click on a combo-box in the current page I have a shot 'black screen' and then the page with the combo-box opened.
Is it possible to avoid this??
Could it be cause by the VirtualKeyboard?

Thanks, regards