Because mIRC itself never sends scroll messages to the MDI client window. It's not up to Khaled to modify mIRC to fix bugs in other software. It's up to your touchpad manufacturer to fix their software to send messages to the right window.
mIRC never sends messages for CTRL+B, either. Nor does it send scroll messages to the input textbox window, yet it seems to have overridden that behaviour nicely. Please read the entire thread before you make another post, because you are asking silly questions.
Besides, if K disabled scrolling of the MDI child window, then people would just complain if they actually want to scroll that and it doesn't work.
Take a look at the video
I posted. Do you think people would complain if they couldn't do that?
If the issue is with my touchpad drivers, then I should have problems reproducing this with a mouse that has mechanisms for sideways scrolling
I'll find one on Monday.
In the mean time, please read the entire thread and watch the videos and consider that I'm asking for an answer from Khaled, not for dribble from you which resembles crap that I've already responded to previously within this thread.