QGLWidget/QOpenGLWidget with transparent background

[FredeHo]

Hi guys,
im trying to get the background of a QGLWidget transparent so I can e.g. draw some Quads, but the background/clearcolor is transparent so one can see the widgets underneath.
This is what ive been trying so far:
1.
setWindowFlags(Qt::FramelessWindowHint);
setAttribute(Qt::WA_TranslucentBackground, true);
(on the QGLWidget)

switching to the new QOpenGLWidget with
setWindowFlags(Qt::FramelessWindowHint);
setAttribute(Qt::WA_AlwaysStackOnTop, true);

But it doesnt seem to work at all.
With QOpenGLWidget I can get a transparent background, but I can't see my opaque stuff as well.
Does anyone have a Solution for that?

I tried this on many different Systems: Win 7/8.1/10 with Qt5.2.1/5.4.2/5.5.0
Best regards,
Frede