OK the first suggestion is, to give user option to scale the UI up for HiDPI screen.
I am using a 4K monitor on Ubuntu and I didn't find a way to scale the UI up. I can switch back to Windows 10 and scale the whole thing there, but still, proper HiDPI support will be much appreciated.
And the 2nd issue might be Linux specific. When running the program in non-fullscreen window mode on Linux, the object rotation / mouse movement ratio seems not correct, and this is not just ArmorPaint, any game built by Armory also have this problem. (note that I am using 200% scaling mode of Ubuntu 17.10)
I will report these 2 issues in Github.