Ok, this is a *possible* bug. It's related only to specific mice and/or mouse drivers, but it only affects mIRC, so I'm going to post it here in case it is an actual mIRC bug.
I just got a new mouse (Microsoft Wireless Laser Mouse 5000) and suddenly I can't scroll very easily in mIRC's channel/query windows using the mouse wheel. If I try scrolling slowly, I can spin that wheel forever without the scrollbar moving, but if I scroll quickly, it works. And it works normally in the Scripts Editor. Again, this only affects mIRC and only the channel/query windows. I'm assuming mIRC handles the scrolling in some custom way rather than using the standard Windows scrolling considering it only affects mIRC.
Now, I'm wondering if this might be related to the mouse having being high resolution (1000 dpi). Maybe mIRC's scroll control is older and just doesn't handle higher resolution mice?
Anyhow, like I said, this may not be a mIRC bug at all. But because it's the only program affected, I thought I'd post it just in case it is.
Btw, I have tried adjusting vertical scroll speed and tried with that speed boost thing, whatever it's called, turned on and off. It doesn't change anything as far as being able to scroll normally in mIRC's channel/query windows.