Thanks. :)
From my observations, sometimes screenshot gets first frame genereted by that OGL window or something from few minutes ago. There must be some buffer, maybe for window miniature, that is used here. Usualy pressing Print Screen twice helps. Exe from Milek7 branch, used in new version 18.01, have build in sreenshot taker under F11. You can also use windows overlay in windows 10, nvidia shadowplay, fraps or something. They take proper frame.