Yes, I do think it's a mIRC bug.
1. The behaviour -only- occurs in mIRC (so I can't reproduce it by say, hovering my mouse near the right hand side of Firefox and doing the same thing).
2. The behaviour -only- occurs when my cursor is within the nicklist zone (so I can't reproduce it by say, hovering my mouse over the buffer and doing the same thing).
3. The behaviour -only- occurs when the nicklist scrollbar isn't present (so it can't scroll).

The video shows where my finger is on the touchpad. It is necessary to prove my point. Your comment was quite useless. What makes you think it -isn't- a mIRC bug?