Baseline (Widescreen) Solution & Issues
Run both the singleplayer executable at least once, to generate config files. Go to \GameData\Demo. Edit jk2config.cfg (preferably with textpad and edit these lines:
seta r_mode "x"
Set this to -1.
seta r_customwidth "xxxx"
seta r_customheight "xxx"
Set this to your favorite widescreen resolution.
Add this line to the bottom of the file:
seta cg_fov "xx"
I recommend 90 for 16:10 resolutions, and 95 for 16:9 resolutions (default is 80).
The wider your aspect ratio, the higher your cg_fov var needs to be to maintain Hor+.
Ultra-Widescreen (21:9) Specific Solution & Issues
Eyefinity / Surround Specific Solution & Issues
4k UHD Specific Solution & Issues
Stereoscopic-3D Solution & Issues