I think I found an issue with the TsScrollbox component. If I set the Max value to something like 128000 and then try to drag the thumb I get an Range Check Error in TsScrollBar.SetPos(). It seems that you are using the old 16-bit windows messages for setting the new position which causes this issue. Using the Delphi TScrollbox works fine with such large values. Is there some way to get around this ?