Assuming you're asking about scroll bars on a computer screen and not trying to dribble a roll of papyrus, the position of a scroll bar is a calculation of a program value scaled to the width or length of the bar. When they bounce, it's usually that the calculation is being restarted or that the scale factor is being changed. It's internal to the software that contains the scroll bar. If it is affecting your ability to use it, contact the people that made it.