YouTube full screen mode only opens halfway

[Alonso Quijano]

Up until this evening, YouTube opened full screen videos as expected on my 15" mid-2015 rMBP (El Capitan 10.11.2). However, now they open as shown in the attached image.

I've tried searching for assistance but no luck.

 

[Jess13]

https://forums.macrumors.com/threads/youtube-problem-in-safari.1943641/#post-22362866