var move = false;
var position;

var object;

document.onmouseup = onMouseUp;
document.onmousemove = onMouseMove;

function onMouseDown(obj,evnt){
  var e = evnt || window.event
  move = true;
  if (e.clientX)  position = e.clientX;
  else if (e.x) position = e.x;

  object=obj;
}

function onMouseMove(evnt){
  var e = evnt || window.event
  if (move) {
    if (e.clientX){
      ResizeListbox(e.clientX - position, object);
      position = e.clientX;
    }else if (e.x){
      ResizeListbox(e.x - position, object);
      position = e.x;
    }
    return false;
  }
}

function onMouseUp(){
   move = false;
}

function ResizeListbox(whereto, ob){
  var begin = document.getElementById(ob).style.width;
  var intbegin = parseInt(begin);
  var newwidth = intbegin + whereto;
  if (newwidth < 100) onMouseUp()
  //Limitamos el resize a 250 como mínimo
  if (newwidth > 250){
    document.getElementById(ob).style.width = newwidth;
  }
}