@Override
    public void draw(Canvas c) {
        super.draw(c);
        if (mFastScrollEnabled) {
            onUpdateScrollbar();
            mScrollbar.draw(c);
        }
    }
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