function mouseDown (e) {
      const off = findPos(bar)
      offset = { l: off.left | 0, t: off.top | 0 }
      clearTimeout(timeout)
      // using setTimeout for visual updates - once the style is updated the browser will re-render internally allowing the next Javascript to run
      timeout = setTimeout(function () {
        setValuesFromMousePosition.call(that, e)
      }, 0)
      // Bind mousemove and mouseup event to the document so it responds when dragged of of the bar - we will unbind these when on mouseup to save processing
      document.addEventListener('mousemove', mouseMove)
      document.addEventListener('mouseup', mouseUp)
      e.preventDefault() // don't try to select anything or drag the image to the desktop
    }
Bind the mousedown to the bar not the arrow for quick snapping to the clicked location.
@param {external:jQuery.Event} e
@returns {void}