Author: Tomas Rutkauskas

I have created my own listbox control as a descendant of TListBox. What I want to 
be able to do is to change the client area of the listbox so that I can draw a 
label above the list box area. I can change the client rect by overriding the 
CreateWnd method like this:

1   procedure TMyListBox.CreateWnd;
2   begin
3     inherited CreateWnd;
4     ClientHeight := Height - 20;
5   end;


But I can't move the client rect down (ie change the origin). Using ClientOrigin.X 
:= 20 does not work as the ClientOrigin property is read only. I'm thinking maybe I 
need to override the CreateParams method to do this. Any ideas?

Answer:

You have to respond to the WM_NCCALCSIZE message:
6   
7   procedure TREDCustomListBox.WMNCCalcSize(var Msg: TWMNCCALCSIZE);
8   begin
9     inherited;
10    Inc(MSG.CalcSize_Params^.rgrc[0].Top, FHeader.Height);
11  end;


That is a the method within my own listbox which does exactly what you want to do. 
Notice how I'm incrementing the client area by the height of the header (which I 
implemented as a separate class so I can do the same thing in other controls). It 
was a fun exercise.

Another clue: You have to paint the header in WM_NCPAINT