The Zircon engine is a touchup of DarkPlaces with 40 small modifications, bug-fixes and slightly polished enhancements similar to the once upon a time "FitzQuake Mark V".
Short list:
1) Streamlined menu with scaling
2) ALT-ENTER fullscreen/windowed mode switch
3) Ctrl +/- adjusts console height
4) Windows scaling fix (so menu is not off-center if Windows scaling is on)
5) FitzQuake noclipping for mapping
6) sv_cheats defaults to 0 for developers/mappers convenience
7) Lower cpu when using the console
8) "folder" "copy" "pos" commands