I've installed the Qt desktop components from git : http://qt.gitorious.org/qt-components/desktop/commits/master and found a few issues with the scrollbar implementation. I tried editing the code in QML and in the cpp files but this is heavily unknown area for me so i.. failed there thus no patch.
The issue if for both the horizontal and vertical scrollbar. Find a long page like http://planetkde.org/ and open it in the Browser.qml demo (in the same git repository). Now you see the site and the vertical scrollbar. (and horizontal, but i focus on the vertical one for now) Now click in the scrolbar on the lower right corner (the bar, not the downwards pointing arrow) and see that you aren't getting a "page down" like effect, infact you just jump to the position where you clicked.
The behavior "should" be on both windows and linux (don't know for mac) that you get a page down effect when clicking far away from the scrollbar handle.
Note: right now when you click anywhere in the scrollbar handle you get jumped to the center scrollbar position. That also isn't the case in windows and linux (again don't know for mac). The case here should be that whenever you cick on the handle (wherever it is) the handle should just be "highlighted" and you shouldn't move to the center of the handle but i suspect the issues are related.