issue with crop system using jquery ui

issue with crop system using jquery ui

So im creating this crop system to crop an image. i haven't coded the
actual cropping part yet just the dragging interface type thing its kind
of glitchy when i drag the box and it's all jittery.. it's kind of hard to
explain so ill just show this jsfiddle
$("#cropper").draggable({
containment: "parent",
drag: function () {
pos = $(this).position();
$('#cropper img').css('left', '-' + pos.left + 'px');
$('#cropper img').css('top', '-' + pos.top + 'px');
}
});