Since QPlatformScreen::topLevelAt() wasn't implemented it would just fallback
to use QGuiApplication's window list.
Possibly fixes any other bug that relied on QApplication::topLevelAt().
The fix is to consult the compositor, which has proper z-order information,
unlike QGuiApplicationPrivate::window_list.
Task-Id: QTBUG-105256
Pick-to: 6.3 6.4
Change-Id: I4202dd5d87e41b69c461c808d29b809af5b52d09
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
40d781b7b5