IIRC (tho I could be making this up), scrollbars became overlay because most mice have a scroll wheel and most trackpads have gesture scrolling support. These a

IIRC (tho I could be making this up), scrollbars became overlay because most mice have a scroll wheel and most trackpads have gesture scrolling support. These are faster than a scroll bar, because you don't need to move your mouse to the side of the screen to use them.