ComboBox stops working when you use fullscreen mode. In this mode, when clicking on ComboBox, the screen just blinks a couple of times instead of expanding the list of values. I have tested this on different computers, with 1 and 2 monitors - the result is the same.
Just try to run the following simple code:
import QtQuick 2.0
import QtQuick.Controls 1.0

ApplicationWindow {
visibility: "FullScreen"
ComboBox

}