It is impossible to do it with QGLWidget without doing a workaround. (QT says it itself, I read it somewhere when I was looking for a solution myself)
I wrote a class (attached) for a European research project and so far it worked perfectly for me.
All you need to do is to subclass it and set the widget that you want to be overlayed as background. (see the main.cpp file I have attached)
Hope it helps!
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