Thanks, that's much clearer - the driver appears to be sending scroll messages to the MDI client window (the window that contains all other windows). There is no reason why it should be sending scroll messages to the MDI client window. There are no circumstances under which mIRC will scroll the MDI client window (the MDI window has that feature by design but mIRC does not use it). My guess is that the driver either does not know how to handle MDI windows or is just confused and is sending the scroll messages to the wrong window.