For example, on Mac, there are no scroll bars visible unless you scroll and it shows the slim little bar.
I beg to differ. The only time the Mac scrollbars are not visible is when Finder windows are sized bigger than the contents on either the x or y axis. In that situation, no scroll bars appear whether you scroll or not. When Finder windows are sized smaller than the content, the not so skinny scroll bars appear whether you scroll or not. And it is the same case in most professional Mac apps. Window and palette scrolling in any Adobe product is a good example. Or Logic Pro X.
