I did checkout your repository and played around a bit with compiling vim with different options, had some trouble getting your MUI-GUI-code to compile (complained about _ParseIFF_v1 or something which I couldn't find any reference to anywhere :/) so I simply compiled it without GUI for now. Well to the point, I actually managed to get the arrowkeys-working in a normal shell, thing is - I'm not exactly sure what I've done :)

Gonna make some more tests and report back here with my findings.

It still can't detect the size of the window though