It can not be the intended behaviour: in many cases you simply can not select the thumb scroller as in any other application

- because you can select it with the cursor not on it (but far below), this happens in full mode
- because when you try to select it (i.e. clicking on it to get 'hold' of it and start scrolling), this does not happen. Instead, the scrollbar behaves as if you where simply clicking on the scrollbar itself (and not on the cursor)

I may make a short video, if it helps.