div태그는 일종의 랩퍼이어서 그 영역사이즈가 변해도 정확한 그 사이즈가 얻어지지 못합니다.

다음의 라이브러리를 이용하면 해결됩니다.

[code]

(function () {
    var attachEvent = document.attachEvent,
        stylesCreated = false;

    if (!attachEvent) {
        var requestFrame = (function(){
            var raf = window.requestAnimationFrame || window.mozRequestAnimationFrame || window.webkitRequestAnimationFrame ||
                function(fn){ return window.setTimeout(fn, 20); };
            return function(fn){ return raf(fn); };
        })();

        var cancelFrame = (function(){
            var cancel = window.cancelAnimationFrame || window.mozCancelAnimationFrame || window.webkitCancelAnimationFrame ||
                window.clearTimeout;
            return function(id){ return cancel(id); };
        })();

        function resetTriggers(element){
            var triggers = element.__resizeTriggers__,
                expand = triggers.firstElementChild,
                contract = triggers.lastElementChild,
                expandChild = expand.firstElementChild;
            contract.scrollLeft = contract.scrollWidth;
            contract.scrollTop = contract.scrollHeight;
            expandChild.style.width = expand.offsetWidth + 1 + 'px';
            expandChild.style.height = expand.offsetHeight + 1 + 'px';
            expand.scrollLeft = expand.scrollWidth;
            expand.scrollTop = expand.scrollHeight;
        };

        function checkTriggers(element){
            return element.offsetWidth != element.__resizeLast__.width ||
                element.offsetHeight != element.__resizeLast__.height;
        }

        function scrollListener(e){
            var element = this;
            resetTriggers(this);
            if (this.__resizeRAF__) cancelFrame(this.__resizeRAF__);
            this.__resizeRAF__ = requestFrame(function(){
                if (checkTriggers(element)) {
                    element.__resizeLast__.width = element.offsetWidth;
                    element.__resizeLast__.height = element.offsetHeight;
                    element.__resizeListeners__.forEach(function(fn){
                        fn.call(element, e);
                    });
                }
            });
        };

        /* Detect CSS Animations support to detect element display/re-attach */
        var animation = false,
            animationstring = 'animation',
            keyframeprefix = '',
            animationstartevent = 'animationstart',
            domPrefixes = 'Webkit Moz O ms'.split(' '),
            startEvents = 'webkitAnimationStart animationstart oAnimationStart MSAnimationStart'.split(' '),
            pfx  = '';
        {
            var elm = document.createElement('fakeelement');
            if( elm.style.animationName !== undefined ) { animation = true; }

            if( animation === false ) {
                for( var i = 0; i < domPrefixes.length; i++ ) {
                    if( elm.style[ domPrefixes[i] + 'AnimationName' ] !== undefined ) {
                        pfx = domPrefixes[ i ];
                        animationstring = pfx + 'Animation';
                        keyframeprefix = '-' + pfx.toLowerCase() + '-';
                        animationstartevent = startEvents[ i ];
                        animation = true;
                        break;
                    }
                }
            }
        }

        var animationName = 'resizeanim';
        var animationKeyframes = '@' + keyframeprefix + 'keyframes ' + animationName + ' { from { opacity: 0; } to { opacity: 0; } } ';
        var animationStyle = keyframeprefix + 'animation: 1ms ' + animationName + '; ';
    }

    function createStyles() {
        if (!stylesCreated) {
            //opacity:0 works around a chrome bug https://code.google.com/p/chromium/issues/detail?id=286360
            var css = (animationKeyframes ? animationKeyframes : '') +
                '.resize-triggers { ' + (animationStyle ? animationStyle : '') + 'visibility: hidden; opacity: 0; } ' +
                '.resize-triggers, .resize-triggers > div, .contract-trigger:before { content: \" \"; display: block; position: absolute; top: 0; left: 0; height: 100%; width: 100%; overflow: hidden; } .resize-triggers > div { background: #eee; overflow: auto; } .contract-trigger:before { width: 200%; height: 200%; }',
                head = document.head || document.getElementsByTagName('head')[0],
                style = document.createElement('style');

            style.type = 'text/css';
            if (style.styleSheet) {
                style.styleSheet.cssText = css;
            } else {
                style.appendChild(document.createTextNode(css));
            }

            head.appendChild(style);
            stylesCreated = true;
        }
    }

    window.addResizeListener = function(element, fn){
        if (attachEvent) element.attachEvent('onresize', fn);
        else {
            if (!element.__resizeTriggers__) {
                if (getComputedStyle(element).position == 'static') element.style.position = 'relative';
                createStyles();
                element.__resizeLast__ = {};
                element.__resizeListeners__ = [];
                (element.__resizeTriggers__ = document.createElement('div')).className = 'resize-triggers';
                element.__resizeTriggers__.innerHTML = '<div class="expand-trigger"><div></div></div>' +
                    '<div class="contract-trigger"></div>';
                element.appendChild(element.__resizeTriggers__);
                resetTriggers(element);
                element.addEventListener('scroll', scrollListener, true);

                /* Listen for a css animation to detect element display/re-attach */
                animationstartevent && element.__resizeTriggers__.addEventListener(animationstartevent, function(e) {
                    if(e.animationName == animationName)
                        resetTriggers(element);
                });
            }
            element.__resizeListeners__.push(fn);
        }
    };

    window.removeResizeListener = function(element, fn){
        if (attachEvent) element.detachEvent('onresize', fn);
        else {
            element.__resizeListeners__.splice(element.__resizeListeners__.indexOf(fn), 1);
            if (!element.__resizeListeners__.length) {
                element.removeEventListener('scroll', scrollListener);
                element.__resizeTriggers__ = !element.removeChild(element.__resizeTriggers__);
            }
        }
    }
})();

// 이용방법

// workspace_container라는 아이디를 가진 div태그의 사이즈 변경이벤트 발생시 처리

function onResize(e) {

console.log(e);

}

window.addResizeListener(document.getElementById('workspace_container'), onResize);

[/code]