protected int getAvailableScrollBarHeight() {
        int visibleHeight = getHeight();
        return visibleHeight - mScrollbar.getThumbHeight();
    }
Returns the available scroll bar height:
AvailableScrollBarHeight = Total height of the visible view - thumb height
android/third_party/recyclerview-fastscroll/src/main/java/com/simplecityapps/recyclerview_fastscroll/views/FastScrollRecyclerView.java