The UX of scrollbars is pretty bad, apart from drag-and-drop. Left-clicking on the empty space more than once quickly switches meaning, as the scrollbar reaches your mouse.
Left-clicking should do what middle-clicking does in KDE, as it is much saner: it scrolls to where you click.
Instead, middle-clicking should systematically go down by a page. It is much more common to go one page down (to continue reading) than it is to go one page up. It should not switch direction as the scrollbar reaches the mouse.