Are you able to reproduce the issue using a certain combination of show/hide/min/max/resize/etc.?
I can't reproduce because it doesn't always happen.
Does it happen after you do something specific? eg. change a system setting?
No, I haven't changed anything.
Did this happen the moment you ran mIRC, at startup? Or was it initially okay and then became like that later on?
When I started mIRC it was normal, and later with mIRC connected to IRC networks, when I noticed the windows were like this, as I showed on the print screen and in the video.
When that happened, I made the print screen and the video, and closed mIRC and restarted and went back to normal.