That is a standard rule in chess. If your opponent has no legal moves (i.e. no way to move without moving his king into check) and is not currently in check, it is considered stalemate, which is a draw.
Have never tried it personally, but this talk made me think collaborative editing is a solved or close-to-solved problem in Emacs: https://emacsconf.org/2023/talks/collab/
Not only did we get a whole new type of Chess engine, it was also interesting to see how the engine thought of different openings at various stages in its training. For instance, the Caro-Kann, which is my weapon of choice, was favored quite heavily by it for several hours and then seemingly rejected (perhaps it even refuted it?!) near the end.