I stumbled upon this and if it is known then I am sorry.
I noticed that Safari had these grey lines down the side of the browser when I was in full screen mode (See Screenshot A)
I wasn't sure what was the cause of it -but after playing with it I found that you could drag the sides to make browser bigger or smaller. just move your mouse pointer to the edge of the screen in full screen mode and you can make the window smaller or bigger. Not sure what the point is to it but hopefully it will help someone
I noticed that Safari had these grey lines down the side of the browser when I was in full screen mode (See Screenshot A)
I wasn't sure what was the cause of it -but after playing with it I found that you could drag the sides to make browser bigger or smaller. just move your mouse pointer to the edge of the screen in full screen mode and you can make the window smaller or bigger. Not sure what the point is to it but hopefully it will help someone