The bug was found on mIRC v7.78.7125 (beta) in Windows 11 23H2.
If open the Windows personalization settings, go to the "Colors" tab and change the default application mode to "Dark", then in the open channel window, in the list of nicknames, both a vertical and horizontal scroll bar will appear, which looks unnecessary, especially when the number of channel users is small. Especially the unnecessary bottom horizontal scroll bar.