I've recently seen latest mac os is having invisible scroll bars which will be visible only once your mouse comes onto it. I think it is a bit tough for the user to know if there is a page overflow or not. Is it not a bad User experience.
I've seen websites employ menu elements (top level) that are invisible until the mouse goes over the menu element. I would suggest that is an example of bad UI, as the relationship to the menu element and the mouse is required and a relationship between the user and the menu element is required for the user to make a decision as to what menu element they want.
When it comes to scrolling, the relationship between the window and the mouse exists whilst the mouse is within the window. It doesn't have to move over to the scroll bar element to invoke the scrolling mechanism. The Apple Magic Mouse, like most modern day mouse devices incorporates a scrolling actions. Magic Mouse is via touch along the long axis of the mouse, others a wheel between the left and right mouse buttons. As such scrolling actions have become second nature in 'window' based systems.
That said, it doesn't mean we should negate the users who have little or no experience of the operating system and Apple particularly fall foul of this. Going for slick interactions over usability.