1.  function onMouseMove(event) {
    2.             moveAt(event.pageX, event.pageY);
    4.             ball.hidden = true;
    5.             let elemBelow = document.elementFromPoint(event.clientX, event.clientY);
    6.             ball.hidden = false;
    8.             if (!elemBelow) return;
    11.             let droppableBelow = elemBelow.closest('.droppable');
    12.             if (currentDroppable != droppableBelow) {
    13.                 if (currentDroppable) { // null when we were not over a droppable before this event
    14.                     leaveDroppable(currentDroppable);
    15.                 }
    16.                 currentDroppable = droppableBelow;
    17.                 if (currentDroppable) { // null if we're not coming over a droppable now
    18.                     // (maybe just left the droppable)
    19.                     enterDroppable(currentDroppable);
    20.                 }
    21.             }
    22.         }

    一开始，currentDroppable是null，判断不相等，然后：
    如果此时球在球门上，则droppableBelow是真，currentDroppable也是真，进入球门
    如果球不在球门上，则没有任何操作，保持原样；

    假设上次进入球门了，currentDroppable是真
    如果此时球在球门上，两者相同，没有变化；
    如果不在了，则离开球门，current变为null，droppableBelow也是null，回到初始状态。