setCapture,captureEvents,releaseCapture 的技巧。

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window.onload=function(){
 drag(document.getElementById('drag'));
};
function drag(o){
 o.onmousedown=function(a){
  var d=document;if(!a)a=window.event;
  var x=a.layerX?a.layerX:a.offsetX,y=a.layerY?a.layerY:a.offsetY;
  if(o.setCapture)
   o.setCapture();
  else if(window.captureEvents)
   window.captureEvents(Event.MOUSEMOVE|Event.MOUSEUP);
  d.onmousemove=function(a){
   if(!a)a=window.event;
   if(!a.pageX)a.pageX=a.clientX;
   if(!a.pageY)a.pageY=a.clientY;
   var tx=a.pageX-x,ty=a.pageY-y;
   o.style.left=tx;
   o.style.top=ty;
  };
  d.onmouseup=function(){
   if(o.releaseCapture)
    o.releaseCapture();
   else if(window.captureEvents)
    window.captureEvents(Event.MOUSEMOVE|Event.MOUSEUP);
   d.onmousemove=null;
   d.onmouseup=null;
  };
 };
}
//-->
</script>