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Settings-apps-all-youtube,force stop and clear data. Restart and open the app again.
If you do not press anything,the video will be on full screen without anything interfering.
I'm not pressing anything. The video is in full screen, but the status bar and buttons bar are superimposed and obscuring the top and right sides of the video.
Those shouldn't be visible on fullscreen,only if you tap the screen they should come up.
They shouldn't, but they are. So the question was what to do.
That solved it. Thank you!
Omg I'm the "have you tried restarting your computer" noob now 
No problem,anytime!