Qt-Painting and OpenGL - is that a problem?

[unusual]

Hallo u all,
I try to bring OpenGL into a given application. The system uses QWidgets as Components. Overlapping or moving a Widget with transparent areas over a QGLWidget works not as expected. The Background is not shining through like on a normal Widget. Is this a default behaviour because of better performance or a matter of principle?
Are there solutions?

greetz

Mike