Draw ?!?!

It does: opposite-colored bishops with no other material is an immediate draw, for example. Appropriately the inverse of normal chess: same-colored bishops cannot mate but can capture each other, while opposite-colored bishops can mate (as someone has to explain every week) but cannot capture each other.

#9 I didn't consider that, but will think about it now.

EDIT: Yikes, the code change to validate this is invasive, so careful testing is required...
[info] Finished in 1 minute 8 seconds, 359 ms
[info] 19 examples, 0 failure, 0 error
[error] Failed: Total 673, Failed 10, Errors 0, Passed 656, Skipped 7
[error] Failed tests:
[error] chess.AntichessVariantTest
[error] chess.HordeVariantTest
[error] chess.AtomicVariantTest
[error] chess.VariantTest
[error] (Test / test) sbt.TestsFailedException: Tests unsuccessful
[error] Total time: 107 s, completed Oct 6, 2018 1:34:17 PM

Good gravy, who wants to code review this? I think it's simpler than the original, but what a change!
github.com/ddugovic/scalachess/commit/d17fea553e52b96be84de778103c9376afa97e0c

Insufficient winning material if the knights are on same-colored squares, in other words insufficient for the player not on move. Seems right, as long as your flag can only be called when it's your turn.

Another weird special case I just thought of: if one of your pieces is a bishop, and all of your opponent's pieces are bishops of the opposite color, then you cannot possibly win, unless it's possible to construct a stalemate with a knight's pawn like lichess.org/analysis/antichess/6bB/6P1/8/8/8/8/8/8_w_-_-
or (with two enemy bishops) with doubled knight's pawns or pawns separated by one file or (4 enemy bishops) two pairs of doubled pawns separated by one file or...

#14 I think(?) I have many but not all of these cases covered, but I may be thinking of something else.

I think you're thinking of opposite-colored bishops + blocked pawns, while I'm talking about lone bishop(s of one color) vs. bishop of the opposite color + any amount of material (non-pawns can only help the opponent win).

As Toadofsky posted in post #4

A lone King can never checkmate against an opponent with at least 1 extra piece of material (Pawn, Bishop, Knight, Rook)
It is a totally unwinnable position, however...

In this case the usual result would be a loss, unless your opponent makes a terrible plunder meaning a stalemate, which equates to a draw.
I have done it in the past (before I registered)
I got in to a very good position and decided to mop up as much material as possible, thinking the win was inevitable, little did I realise that a blunder meant a stalemate and a draw, from a position I should have won so easily.

@Toadofsky Concrete example of an incorrect loss on time:

There’s barely anything correct with those „variants“. Could we stick to chess? Thanks.