how to move div up & down on window scroll

You should be using 'position: absolute' and 'top' and 'left' instead of margins. By using margins you are pushing them off each other making them make the page massive.

$(window).scroll(function() {
  $(".mydiv").css({
    "top": ($(window).scrollTop()) + "px",
    "left": ($(window).scrollLeft()) + "px"
  });
});

See this codepen - http://codepen.io/dmoojunk/pen/JXBaXm