quoting Kereb:
yeah they also sometimes have a way of breaking the use of the up and down buttons for scrolling
EVEN when you click outside the video on some nice safe whitespace ... in the most hilarious version of this, you can sometimes be using the down key to scroll a page and without clicking anything the down key will suddenly stop scrolling the page when the youtube window becomes visible and it'll just start turning down the volume in the unclicked, unplayed youtube clip
The arrow key issue sounds like something Only On Mac®. What browser are you running?