Let me summarize the issue and I will then lock the thread since it has gotten somewhat out of control.

Situation: a mouse/pad driver is sending scroll messages to mIRC and, for some reason, it is sending them to the MDI client window. The MDI client window then reacts to the scroll messages and scrolls the windows in the MDI client window. This behaviour has been reported as a potential bug.

As far as I can tell, the MDI client window is behaving correctly. It should scroll if it receives a scroll message, in the same way that the nickname list or text display should scroll if they receive a scroll message.

There are hundreds of standard messages that can be sent to the MDI client window, as well as all other controls, that could manipulate it in many other ways. An application like the mouse/pad driver could choose to reposition or resize windows, change their colors, or even hide them completely, using standard windows messages. While mIRC could choose to actively ignore and block hundreds of messages from other applications/drivers, that just would not make sense. It is up to the external applications/drivers to know what they are doing to other applications on your system.

That said, I am still open to the possibility that this is a bug in mIRC, however based on what I have read it seems very unlikely. Thanks for the feedback everyone.