Unnecesary redrawing when child window titlebar gets dragged under parent window titlebar

I discovered this with the multiwin application, and didn't test with any
others. When one drags the child window to the top, let the mouse cursor end up
around the minimize, maximize and close button (close to it, though entirely at
the top doesn,t seem to be required, at least the minimize and maximize button
get redrawn, while nothing was ever over them, so there's absolutely no reason
to do so.