The open source Zircon engine ("DarkPlaces Mark V Project") is a modification DarkPlaces engine with a mouse driven menu everywhere, ALT-ENTER fullscreen/windowed switching and 88 other enhancements to support user/mapper/mod coder ease-of-use and optional conveniences for those interested in a "1996 Quake" experience. "csqc_full_width_height 1" in console when running Arcane Dimensions at this time It is hoped that some/many/all of these enhancements will eventually make it into official DarkPlaces. The Zircon engine seeks to keep full compatibility with DarkPlaces so either engine can be used interchangeably.
Tomb of Thunder v_cshift issue that results in all black screen that never clears has been eliminated (this bug likely affects every DarkPlaces going back a very long time). Empty submodel warning is now a developer print, as are division by zero Quake messages -- other engines don't print those and while I think they are nice, not appropriate if you playing a mod (not developing one).