Well that was quick!
I can confirm it works with this situation I invented: FEN:
I think the “X” is convention and works well graphically so don’t worry about that.
I can’t test the animation because I don’t have the mobile version (one of the few people that don’t own a mobile).
The only suggestion for improvement I’d have is that it would be nice to have the en passant automatically be placed when you move a pawn up 2 spaces but that might be asking too much. I ask this because I read in the Wikipedia article on FEN that: “If there’s no en passant target square, this is “-” [as you already have implemented]. If a pawn has just made a two-square move, this is the position “behind” the pawn. This is recorded regardless of whether there is a pawn in position to make an en passant capture”. At any rate, it works as is!
Love the forums too BTW very clean, it’s a pity not more people have signed up.