Baseline (Widescreen) Solution & Issues
Resolutions are listed in-game but 3D elements and some 2D elements stretch from 4:3. 2D elements that maintain the correct aspect ratio do not increase in size at higher resolutions.
Ultra/Super-Wide (21:9/32:9) Specific Solution & Issues
Same as widescreen (stretched from 4:3).
Eyefinity / Surround Specific Solution & Issues
Same as widescreen (stretched from 4:3).
Multi-monitor setting in launcher has no effect on aspect ratio behaviour (seems to be merely for choosing which display to use).
4k UHD Specific Solution & Issues