One modification to this game I'd like to make is adding the missing knook piece.

But sadly I don't think I'm up for it: Do not expect the code to be stable under any modifications, but you can try if you want I guess.

Build the board out of bricks and it's both stable amd you get easy access for bricking online!

What's a knook? A mix between knight and rook? How would it work?

Yup. It’s a piece that can move anywhere a knight or a rook could move. Invented (I think?) by /r/AnarchyChess

I've known this as the "Empress", a fairy chess piece.

That sounds more like queen + knight

Looking at the code adding another piece should be doable since I don’t think any pieces call other pieces attributes, and the pieces at game start are just objects in a list…

Adding the knook would be pretty simple actually. I'd just need to combine the code for the rook and the knight and then make a function to snap the mouse position to the right closest point on the crosshair shape.

It’s pretty fun surprising someone who is not familiar with it, in a sadistic sort of way.

Also true for some other little rules people often forget, like underpromotion, castling through/out of check, stalemate, perpetual, 50 moves, etc.

Plus the general topic of how to use clocks.

Chess is so popular that most people first learn to play from parents, relatives, etc., so they might not have learned the full rules if they aren't interested in learning the game more seriously.

You say underpromotion, but something people are surprised by is making a second queen ime. People sometimes assume promotion is actually "taking a captured piece back".

Very understandable how this developed in the era of over the board play. You can't have two queens when they don't physically exist!

I always use an upside down rook, somehow there's always one available by then.

I end up using a checkers piece, since pretty much every board I've played on has them.

It used to be a potential invitation for a fist-fight before we had pocket computers with which the aggrieved (and unaware) player could verify the move's legality.

This is true.

I've taken to asking players who are unfamiliar with the game if their familiar with it on occasion, at least ones who I expect will be more frustrated than amused.

I was playing in our company chess tournament one year and played en passant against an opponent who entered but didn't play much chess.

I swear he thought I was cheating. I had to explain the term and have him Google it.