When playing Stk, I have a second monitor that I use for other stuff, notably online while waiting that everyone presses Ready in a Lobby. It used to be fine but now, Stk minimizes when it loses focus, which is annoying as I cannot read the Lobby's messages and need to restore the "window" each time to continue playing.
Play in Fullscreen on 1 monitor out of 2 or more, make Stk lose focus by doing something with the other screen.
There is no issue if playing windowed.
STK release version: 1.2, maybe a few commits later...
STK Source (PPA, distribution package, official binary, etc.): GitHub
System: Debian 10
Graphics card: Rx 560
CPU: Ryzen 7 3700X
..:: Antarctica Rendering Engine 2.0 ::..
[info ] IrrDriver: OpenGL version: 4.5
[info ] IrrDriver: OpenGL vendor: X.Org
[info ] IrrDriver: OpenGL renderer: Radeon RX 560 Series (POLARIS11, DRM 3.36.0, 4.19.0-10-amd64, LLVM 7.0.1)
[info ] IrrDriver: OpenGL version string: 4.5 (Core Profile) Mesa 18.3.6
try
if (CreationParams.Fullscreen)
SDL_SetHint(SDL_HINT_VIDEO_MINIMIZE_ON_FOCUS_LOSS, "0");
in CIrrDeviceSDL.cpp 
That seems to work fine :D !
I'm not near the computer atm but it should be possible to use
export SDL_VIDEO_MINIMIZE_ON_FOCUS_LOSS=0
before launching STK, so no need to recompile
Lol fouks should know that invulnerbility cannot exist in stk online design! 
Oops look the wrong column names sorry, someone reported fouks "invulnerbility hack?" lol