Overlaying QGraphicsView (QGLWidget) with layout results in intransparent layout

Hi,

I'm using a QGraphicsView with a QGLWidget viewport:

setViewport(new QGLWidget(QGLFormat(QGL::SampleBuffers)));
setViewportUpdateMode(QGraphicsView::FullViewportU pdate);

When overlaying the QGraphicsView with a layout like (QHBoxLayout, QVBoxLayout, QGridLayout or QFormLayout) the layout rect gets visible and is filled with a strange pattern (See Attachment).

I'm using Qt 4.5.1. Is there anything I can do to get a transparent layout on top of a QGraphicsView (QGLWidget)?

Thank you in advance

Markus