Google is currently experimenting with a slick new transparent player. If you want to activate it for yourself, you can do so with a simple cookie edit.
The new transparent player is mostly an aesthetic upgrade to YouTube (although, after years with the same player, it's more than welcome), however it comes with one feature that most other players have had for a while: it disappears while you're watching a video and only re-appears when you hover over the window. If you want to try it out, you just need to edit a single cookie. Chrome users will need an extension like EditThisCookie to modify it. Install the extension, open up YouTube, and click the extension icon. Then, find the following cookie:
Then, change it's value to the following:
Firefox users can do the same thing without an extension using the developer console. Just press Shift-F2 while on YouTube and follow this guide to edit your cookies.