At design time, the form looks and acts like any other form. I have made the form colour of green, just to make it a bit loud...

At runtime though, the controls sneak into their parent window and modify its region...

Umm... I seem to have had a code loss at some point between machine changes etc. The file that thinks it is the control code is not at all the control code. I will have to look for it some more and/or remake it.

The basic gist is that the panels, maybe in OnShow, though OnCreate MIGHT work, do a GetWindowRgn from their parent, then either add or subtract their rectangle from the parent's region then reset the parent's region to the new region. Each panel does this in turn and the resulting form is holed appropriately. I seem to recall needing to mess about with borders on the panels last I looked.

Sorry about losing the code. I'll add it to my list of things to do. Anyway, you should be good enough by now that you could write it yourself.