Qt Quick needs a way to enumerate QScreen instances in a QML-ish manner, and a property on Window to associate with the chosen screen. This would map to QWindow::setScreen() of course.
This is an issue for QtWayland where compositors written in QML - combined with a focus on multi-display in the high-end embedded space - is one of the key selling points. Others are affected too, e.g. Quick Controls promotes the usage of QQmlApplicationEngine and the Window element. It is currently impossible to utilize multiple screens without resorting to C++.
Note that the existing Screen attached property is useless for our purposes here.