Baseline (Widescreen) Solution & Issues
Resolutions can be changed in DATA/ufo-et.ini (if not listed in the options menu)
width=
height=
There are stretched 2D screens: inventory, base, ufopaedia... etc (game menus are stretched as well). Gameplay is not hurt by being stretched on these screens. There is no "mission map" where the scale would matter and stretching would throw it off.
Ultra/Super-Wide (21:9/32:9) Specific Solution & Issues
Eyefinity / Surround Specific Solution & Issues
4k UHD Specific Solution & Issues