не корректная работа getComputedStyle

<div id='dialog' style="position:fixed; left:50; top:50; border: 1px solid; box-shadow: 0 5px 10px rgba(0, 0, 0, 0.8); z-index:1000; ">
            <div style="padding: 10px; background: #FFFFFF;">
                <div class="content" id="cont" style="overflow:auto;padding: 5px;height:100%;">HELLO WORLD</div>
                <div style="text-align:right;width:100%;"><img src='s.png' id='res' style="cursor: nwse-resize;"/></div>
            </div>
        </div>

           var res = document.getElementById('res');
           res.ondragstart = function() { 
                    return false; 
            };
            res.onmousedown = function (e) {
               var d = document.getElementById('dialog');
               var computedStyle = d.currentStyle || window.getComputedStyle(d, null);
               d.style.height = computedStyle.height;
           }