scene graph not initialized on custom window - results in garbage window

As there is no native-qt-way to set advanced window types/characteristics (like shown on all virtual desktops/window type panel/desktop etc)
I use kwindowsystem, part of KF5(KDE Frameworks 5). Custom Class inheriting QQuickWindow and getting it's custom properties assigned there results in:
a) window is garbage when color is set transparent/anything with alpha channel (with NVidia driver)
b) window get's color black instead of specified transparent color, but window is not garbage (with nouveau driver)
c) color is set to anything with no alpha channel (like red,blue,green etc)
works, but you can't set it to transparent.

I found out that the class (when initially color is set to transparent) is never initializing the scene graph - the QQuickWindow::sceneGraphInitialized signal is never emitted)

Example attached, libkwindowsystem-devel required.