After a while (now) it turned out that this window would also make a very nice control element for a window of another application I'm, working on.
Depends on how much clean solution you need.
First of all, despite being derived from TopWindow, you can still use such class as child Ctrl.
OTOH, usually it is very simple to combine child Ctrl with TopWindow (MyCtrl c; TopWindow win; win.Add(c.SizePos()).
Sometimes an interesting solution is to add TopWindow as Ctrl's option - at first, it might look a little bit strange (because C++ and GUI containment are reversed):