It is currently possible to middle-click in the scrollbar gutter to make the thumb jump directly to that point.
It turns out that I use this more than I thought I did!
I use it for long documents; if I want to jump to the end of a long document, I mouse over the bottom part of the gutter and middle-click to jump right there.
With the new ayatana scrollbars, I have to mouse-over the thumb where it currently is, and then drag the thumb to the bottom of the gutter.
The process used to be: mouse to bottom of gutter (one mouse move, and the bottom of the gutter is in a fixed position in the window), middle-click (one mouse-click), done.
Now it's: look for the thumb (which requires thought), mouse to the thumb so the big thumb appears (one mouse move), drag the thumb to the bottom of the window (one mouse click, followed by another mouse move). And because you're not going straight from point-to-point, but going through all the points in between, it's also slower because the application needs to update the view of the document many times as you're dragging.
See the attached video for a graphical demonstration of the problem.
Sadly, I don't have a good proposal for fixing this. :(
The main problem for this is discoverability. Given the small number of people who might use it, and miss it, it could be added as patch (not by default).