Splitting this out from bug #234871. Scrollbar steppers are mispositioned if the theme enables steppers. See steps to reproduce from bug #234871.
Created attachment 451688 [details] Screenshot demonstrating bug With the GTK scrollbar (on the left), the top stepper is horizontally centered above the slider. With the WebKit scrollbar (on the right), the top stepper is clearly mispositioned, too far to the right of the scrollbar rather than centered.