code: private void slideKnob(Event event) {
  		int x = DOM.eventGetClientX(event);
  		if (x > 0) {
  			int lineWidth = lineElement.getOffsetWidth();
  			int lineLeft = lineElement.getAbsoluteLeft();
  			double percent = (double) (x - lineLeft) / lineWidth * 1.0;
  			setCurrentValue(getTotalRange() * percent + minValue, true);
  		}
  	}
docs: Slide the knob to a new location.
  
  @param event
  the mouse event