Author: Tomas Rutkauskas

Is it possible to define (set the size and position) of a window's client area 
(without resizing the window itself)? What I want to do is increase the non-client 
area to get more space to paint my own custom borders around a window. I want this 
to be reflected to the client area so that my border is protected from anything 
that goes on in the client area (painting, scrolling etc.).

Answer:

You have to handle the WM_NCCALCSIZE message on your form. See win32.hlp for 
details. The following example handler for a TListBox descendent excludes some 
space for a header bar from the listboxes client area:

1   
2   procedure THeaderListbox.wmnccalcsize(var msg: TWMNCCALCSIZE);
3   begin
4     inherited;
5     if msg.CalcValidRects then
6       with msg.CalcSize_Params^.rgrc[0] do
7         top := top + Itemheight + 4;
8   end;



I hope you know how to define a message handler in the class declaration.