If this isn't mg-desktop, then what is it?

That viewer you see is the mg-desktop map viewer component refactored to work against our MaestroAPI classes instead of mg-desktop (namely the RuntimeMap class of the MaestroAPI), meaning we now have a 100% purely managed map viewer component that you can use for your own applications that use the MaestroAPI.

Or in our case, the centerpiece of a new feature coming in the next beta of Maestro: A standalone "live" Map Definition editor.

If you've played around with the mg-desktop viewer, the Maestro map viewer is 90-95% identical to the mg-desktop one in terms of functionality. It is basically a super-rich version of the AJAX viewer.

As to the editor itself, it's called a "live" editor because what you see on the map is what is currently being edited. Layers that are added to the map and any structural and draw order changes are reflected immediately in the viewer. This editor will not fire off any web browsers for previewing because you are looking at the final result! In a way, it's similar to the old MapGuide Author 6.5 and its way of authoring maps.

This viewer still currently has the same limitations as the mg-desktop one, that is: no tiled map support. If you do open a Tiled Map Definition, the editor will give you the opportunity to convert non-tiled layers to tiled ones, which you will have to then convert back with the traditional Map Definition editor when you're done. So if you primarily create and work with lots of Tiled Maps, this editor is probably not for you (for now).

Needless to say, this feature is completely experimental and full of un-tested corner cases, so put on your beta testing cap when playing with this new feature in the next beta of Maestro. And if I haven't made it clear already, this is a standalone editor. The existing Map Definition editor is not going away.