Thank you for playing, I am glad you found it interesting. :) I might make a sequel in the future, using suggestions from (so far two) other comments to make the movement more open and it would include more variety. In this one, I was primarily experimenting with my new gameplay mechanics idea.
I know about the browser window issue, I forgot to add the e.preventDefault() and e.stopPropagation to keydown event handler and only remembered about it when the submission deadline was over and I was not sure if it is okay to re-upload after that while the voting phase is still ongoing (I also wanted to increase music volume from 0.25 to maybe 0.5 (after hearing that it is almost inaudible in some setups while sound effects can still be heard clearly), but decided against it (at least until the voting phase ends) for the same reason). The issue seems to be browser dependent (it somehow does not happen in Firefox, only in Chrome (and probably Chromium-based browsers in general)). The workaround is to play in full screen or use WASD controls (or a gamepad).