Reply to a message, make a move, submit move.
Works fine unless the move you make is illegal, then you get a half-blank screen and no indication of game status. Go back to MyGames and find that the game hasn't gone anywhere and your message reply didn't happen either. Do this several times until you figure out that the problem is an illegal move. Annoying.
Better option: still stay on the game-playing page, but display the board with the illegal move squares highlighted. I think this already happens if there isn't an outgoing message.