Transparent X11 window with Qt::FrameLessWindowHint is solid black
As per the title, when creating a maximised transparent X11 window using Qt5 with Qt::FrameLessWindowHint set, Mutter (Wayland) renders a solid back window. Not sure whether this is the actual window or a helper created by Mutter. The black window has a WM_NAME of "mutter guard window". The attached test case demonstrates this (see the ReadMe.txt for build instructions). See also the thread on gnome-shell-list. Thank you Jonas and Olivier for investigation so far.