QQuickFramebufferObject: where is the custom FBO used ? How can this cause flickering? - qtquick2

Question

I am currently integrating a custom OpenGL viewport within QML via QQuickFramebufferObject. I observe some flickering of the whole app window (not only of the QQuickFramebufferObject window) and I am wondering why.
My custom QQuickFramebufferObject::Renderer does create a separate FPO:
QOpenGLFramebufferObject* OsgRenderer::createFramebufferObject(const QSize &size)
{
QOpenGLFramebufferObjectFormat format;
format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
return new QOpenGLFramebufferObject(size, format);
}
So I understand that Qt defines its own FBO type (QOpenGLFramebufferObject) so it can later use the FBO to mix it with its own GL rendering, is that right?
I then do all my rendering
void OsgRenderer::render()
{
assert(m_osgItem);
if ( !m_osgItem->getViewer() )
return;
// Without this line the model is not displayed in the second
// and subsequent frames.
QOpenGLContext::currentContext()->functions()->glUseProgram(0);
// Ask OSG to render.
m_osgItem->getViewer()->frame(); // WARNING: non-blocking (executed in a thread of its own - in a thread-safe way).
// Reset OpenGl state for QtQuick.
m_osgItem->window()->resetOpenGLState();
}
I expect QtQuick to actually gather my custom rendered FBO with its own rendering of the window after the call to render(), is that right?
My rendering is actually achieved in a separate context. In this context, how can it cause flickering? Any idea?