Created attachment 217557 [details]
Black king castling with white bishop, glChess 3.4.1

glChess 3.4.1 allows short and long castling with opposing bishops and knights.

Attached, is a PNG showing a sequence of movements played in the stated version that allows the black king to castle with the white bishop; note that the PGN ends with a 0-0 on black's side. A similar sequence allows castling with the white knight, and likewise for long castling, or for white king castling with black pieces. Castling with opposing rooks and queens is not possible in this scenario because they put the king in check; interestingly, the opposing king does not allow castling, long or short, in these circumstances.

The condition checks should probably make sure that capturing the rook disables castling as much as moving it.