Just tested it out and I experienced the same problem of invisible borders being very close to starting position (not the whole window) same as de_jean_7777 reported. I'm using Google Chrome. But when I refreshed the web page using F5 key the invisible borders disappeared and I could move across the entire window.

After some further testing I noticed that I if I open your link using right click and "Open in new tab" I always end up with invisible borders. But if I open your link normally with left click (still opens a new tab for me) there aren't any invisible borders present.
So I'm assuming that when you open some link in new tab Chrome actually does some re sizing of web window area which then results in smaller game area being set as the web window dimensions are retrieved to quickly.