tenchi71, we don't autoplay videos, that is controlled by the sites. We try to avoid letting videos to autoplay when the tab is in background, apparently there is some case on the site you visit that we don't catch properly, could you provide us the address to it?
The automatic full screen, is something we have added for convenience. We are aware of some problems connected with it -- and I really understand your frustration if it auto-starts in full screen from a background tab. That is definitely not intended behavior!