The attached example (opengl_transp_mdi.cpp) demonstrates an issue which is that when drawing OpenGL objects with transparency inside widgets that are inside an MDI area, weird artifacts are visible when moving around the windows. To reproduce, compile the attached code, then take one of the sub-windows and move it around: ghosts of other windows will be visible through the lines (I have attached a screenshot too).

Note that this only happens if the main window/MDI area is shown before the sub-windows are added. In my application, I need to add the sub-windows on-the-fly, so this is why I need to do it this way.