OpenGL widget with winId in floating/maximized DockWidget causes corrupt rendering

We need to get an OpenGL window handle in order to draw third party video content. We do have QDockWidgets (with our own TitleBar) to undock and maximize these widgets which are inside a grid layout. If the DockWidget is docked everything is fine, but if it is floating, we experience strange behaviour. We broke the issue down into a sample project with just a QDockWidget with maximize button and two widgets from the Qt OpenGL example '2DPainting'. See attached screenshots from docked/floating/maximized states of the DockWidget and its content.