Unless you're seeing something different, the tiny space is there also when scrolled to the top and can also be seen in any window in mIRC. I also just tested in Firefox and saw the same tiny space. It looks like it's just a Windows scrollbar display thing rather than something in mIRC. It's the same in mIRC 6.35.
If you're seeing something different than that 1mm (probably more like .5mm) space between the bottom of the scrollbar and the top of the down arrow on the scrollbar, then please post a screenshot because I'm not seeing that.
