On OS X, the default behavior hides scrollbars in several different circumstances. I can't stand it and I normally have them shown all the time.
The issue for me was that I wasn't expecting to scroll in a game. Nothing against scrolling in games, but it's not the paradigm I'm used to.
Maybe move the buttons "above the fold"? It kind of messes up your current layout, but I wouldn't want other people to miss playing your game because – like me almost – they didn't see the start button. Anyway, just a suggestion. Cheers.