Consider a main window with dock widgets in top and bottom dock areas, and no central widget. It's possible to grab the splitter handle above the bottom dock widget, and drag it upwards over the top dock widget, where it will draw itself on top of the widgets in the top dock widget. This doesn't happen if there is a central widget.

This same problem occurs with left/right dock widget areas.

Note: please use a central wigdet. QMainWindow never worked correctly without one.