Alright, I can't really call this a GUI but yeah, it's just a slight improvement from youtube's side, I remember using the same effect a few months ago with a plugin for chrome (I have forgetten which one it was,Derp) but now its officially released by Youtube! Well, I'm not even sure if this is a permanent change or temporary testing by youtube but it does look really cool!
A view at full screen: Screenshot by Lightshot
You can also enable or disable it by taking a look here. https://www.youtube.com/testtube
I'm not sure if this is still some aftermath of the add-on I was using but to confirm that, Have you guys gotten the new Youtube design yet?
Also the stop and play button popup animations are really sick!

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isn't new yet , already posted https://a2s.in/check/1617-feature-youtube.html

btw congrats for the mod tag