Black horizontal line in Safari new tab bar when full-screen

I think this started a few new operating OSX's some time ago. I always used to have a certain page in full screen and then just put my cursor above the top to reveal the toolbar, but nowadays the black Safari toolbar just ruins the view on a page where I do not need or want it.

The previous thread on this a couple of years ago got nowhere if I remember correctly. A lot of stuff around safe mode, but nothing that resolved the problem. I only need it on one page, however, I would like to be able to have a full page without the black Safari bar showingng.