The arrow keys scroll up and down. To prevent this, add this line in your event listener, where you handle up/down arrow events:
Also please check out my game at https://repl.it/talk/challenge/Space-Runner/12625.
Very cool game. Here is the score board API, adding it to your game will make it much more fun! : https://repl.it/ibuiltthis/games/p/attention-scoreboard-api