I had great difficulty using the scroll bars with TheBrain Android and via the Android web browser on my Samsung Galaxy S5. I use my finger and a smartphone stylus.
However, last week I found a narrower stylus that is still designed for use on this smartphone. It is the Targus "slim stylus". It is about 75% the diameter of my initial stylus and far more precise. I can now scroll in both TheBrain Android and my Android web browser (accessing my web brain) on my Galaxy S5.