I'm sure there is more to it than just an issue with mIRC. I can drag anything across mIRC and never see any graphics problems. This is true both in XP and Windows 7 (32bit each) and although I don't have Vista to look at right now, I never saw it happen when I used Vista.
That said, it could still be caused by mIRC, but only happens with certain system setups. I rarely use the newest graphics drivers and there are times with the newest drivers have issues. It's even possible for both ATI and nVidia drivers to have a similar issue in certain versions. If the drivers are related, then it wouldn't matter which card you're using since they both (at least nVidia) have driver packages that handle all their newer cards rather than individual drivers for each card. Drivers would also make it occur in any system using that driver version. Whether or not that is the case, you'd have to see. Try older versions of the drivers for the cards and you'd have a good idea if it's driver related.
Either way, it's going to be related to something on the tested systems that is different from the 3 systems I have available to test on. mIRC by itself won't display that bug from what I can test. If mIRC has a bug that is related, then it's only there if some other requirements are met -- drivers (not just graphics), hardware (not just graphics), other software being used, Windows settings, etc.
And considering mIRC's been out for a many years, if this were a widespread problem, I'm sure it would have been brought up here many times before now. That leads me to think it's probably a very limited problem that may have nothing to do with mIRC itself. Remember that mIRC draws windows differently than most other programs, so just because it doesn't happen in other programs doesn't mean it's a problem with mIRC. It only means that it's only visible in applications that draw the windows in this specific way.
