Safari Youtube full screen border flicker

Whenever I open YouTube in Fullscreen I have a border on the top and bottom of the video that looks like a faded version of the Wallpaper and flickers on and off. Whenever the controls and mouse are showing a black border is shown as is expected, the problem arises when the controls are automatically hidden after x seconds. I tried to right-click the video twice and select safari's own Full Screen, but the same problem appeared. I have attached a picture showcasing the problem below. I had to take a picture of the screen because the problem does now show when taking a screenshot.

Thanks in advance for your help.

System Details

Browser: Safari 11.1

OS: macOS High Sierra (10.13.4)

MacBook Pro (Retina, 15-inch, Mid 2015)

Processor: 2.2 GHz Intel Core i7

Memory: 16 GB 1600 MHz DDR3

Graphics: Intel Iris Pro 1536 MB

Picture demonstrating the Problem