How to get rid of annoying YouTube sidebar

When in YouTube on Safari (11.0.3, macOS 10.13.3, new iMac) how can I permanently get rid of the annoying YouTube sidebar without having to click on the little grid at the top left:

I would like YouTube to always look this, i.e. without their sidebar:

I can´t find any adjustment or preferences for this, but I´m probably missing something! Thanks for any help/suggestions! Ian.