QGLFramebufferObject does not reset OpenGL Viewport

When a Framebuffer Object is used for rendering a Qt-Widget into a texture during OpenGL Scene rendering, the openGL Viewport remains set to the size of the framebuffer object.

I am using the FBO like this.
1. Initialization :
fbo = new QGLFramebufferObject(renderWidget->size(),QGLFramebufferObject::CombinedDepthStencil,GL_TEXTURE_2D);

2. during OpenGL rendering
// here viewport is (0,0,1280,800) = Canvas size
widgetToRender->render(fbo); // widgetToRender is a QWidget*
// here viewport is (0,0,1024,768) = size of widgetToRender

by reading and setting the vieport after widget rendering the problem can be resolved, but i would expect the opengl state to be the same before as after rendering using a frambuffer (this was the case prior to Qt 4.6.)