mIRC does not process scroll messages for the MDI client window. All Window controls process standard messages by default, including scroll messages. What is happening is that the driver is sending scroll messages to the MDI client window and the MDI client window is then doing what it should be doing - if it receives a scroll message then it should scroll the MDI client area and all windows in it. This is correct behaviour and I would expect it to do that.

The question is, why is the driver sending scroll messages to the MDI Client window and not the active window with the focus. That can only be answered by the developer of the mouse/pad driver I'm afraid.