Author: Jonas Bilinkevicius

I'm using a form with a TPaintBox element that exceeds the size of the form, so the 
form has two scrollbars. I want the user to be able to scroll the form using the 
keyboard (with cursor keys). How can I perform scrolling programmatically? I tried 
using the TForm.ScrollBy method, but the results are a bit strange.

Answer:

Do not use ScrollBy, instead send WM_VSCROLL messages to the form to make it do the 
work for you.
1   
2   procedure TfrmMain.FormKeyDown(Sender: TObject; var Key: Word; Shift: TShiftState);
3   begin
4     case Key of
5       VK_DOWN: {scroll down}
6         begin
7           Perform(WM_VSCROLL, SB_LINEDOWN, 0);
8           Perform(WM_VSCROLL, SB_ENDSCROLL, 0);
9         end;
10      VK_UP: {scroll up}
11        begin
12          Perform(WM_VSCROLL, SB_LINEUP, 0);
13          Perform(WM_VSCROLL, SB_ENDSCROLL, 0);
14        end;
15    end;
16  end;


If you use ScrollBy you also have to manually adjust the scrollbar position since it only scrolls the windows client area, completely independent of any scrollbar.