This not a bug. This is a request for a new feature.
When I want to scroll through a large document, I use the wheel of my mouse. This is the easiest way for little moves around the current position on the document.
So, when I click to the scroll-bar, I don't want 'Page_Up'/'Page_Down'.
I would like a direct jump to the location pointed by my cursor, centered around it.
Page_Up/Page_Down could be affected to the buttons on the top/bottom of the scroll bar (instead of line_up/line/down), when they exist.
What do you think about that?
This is out of scope of the window manager.
*** This bug has been marked as a duplicate of bug 379498 ***