ScalaJS removeEventListener上的draggable不工作

基于js演示: http://jsfiddle.net/wfbY8/737/

class Draggable(element: HTMLDivElement) {

  var offX: Double = 0
  var offY: Double = 0

  val divMove = (e:MouseEvent) => {
    //element.style.position = 'absolute';
    element.style.top = (e.clientY-offY) + "px"
    element.style.left = (e.clientX-offX) + "px"
  }

  val mouseDown = (e:MouseEvent) => {
    offY = e.clientY - element.offsetTop
    offX = e.clientX - element.offsetLeft
    window.addEventListener("mousemove", divMove, useCapture = true)
    println("added")
  }

  val mouseUp = (e:MouseEvent) => {
    window.removeEventListener("mousemove", divMove, useCapture = true)
     println("removed")
  }

  def addListeners(){
    element.addEventListener("mousedown", mouseDown, useCapture = false)
    window.addEventListener("mouseup", mouseUp, useCapture = false)

  }

  addListeners()

}

从客户端代码来看,我使用它的方式如下:

  val panelElem = document.getElementById(COMPONENT_ID).asInstanceOf[HTMLDivElement]
  if (draggable == null) {
    draggable = new Draggable(panelElem)
  }

我在日志中看到“添加”和“删除”。但元素仍然可以移动(当我移动鼠标而不按下它时),就好像我没有从侦听器中删除mousemove事件一样(在mouseUp上)。

我想知道为什么。。