window.onload = addListeners;

​

function addListeners(){

    document.querySelector('.resize').addEventListener('mousedown', mouseDown, false);

    window.addEventListener('mouseup', mouseUp, false);

​

}

​

function mouseUp(){

    window.removeEventListener('mousemove', divMove, true);

}

​

function mouseDown(e){

  window.addEventListener('mousemove', divMove, true);

}

​

function divMove(e){

​

  var div = document.querySelector('.resize');

  div.style.position = 'absolute';

  div.style.top = e.clientY + 'px';

​

  var altura = div.previousElementSibling.getBoundingClientRect().top;

  div.previousElementSibling.style.height = (e.clientY - altura) + 'px';

​

}