test jquery animation start width/height

start width and height of box-sizing: border-box are measured incorrectly during animation

HTML

<div class='testbox' style='box-sizing:content-box;top:20px;left:10px'>content-box</div>    
<div class='testbox' style='box-sizing:border-box;top:20px;left:200px'>border-box</div>
<input type='button' id='test' value='test'/>

CSS

.testbox {
    position:absolute;
    width:50px;height:50px;
    border:10px solid red;
    padding:10px;
}

JavaScript

function test() {
    var size = Math.round(Math.random() * 20 + 60);
    $('.testbox').animate({
        width: size,
        height: size
    });
}
test();
$('#test').click(test);