While sliding it from 0% to 100%, the 'thumb cursor' reaches 100% before the mouse pointer does, after some strange animation 'jumps' which have no bearing on t

While sliding it from 0% to 100%, the 'thumb cursor' reaches 100% before the mouse pointer does, after some strange animation 'jumps' which have no bearing on the step value setting. Also, clicking anywhere on the bar doesn't set the slider to the centre of the click. They don't appear to be making the correct calculations based on Bounding Client Rects and Offset widths.