Edit in JSFiddle

<div class="container">
    <div style="position:absolute; top: 0; bottom: 0; left: 0; right: 0; z-index: 2; overflow: auto;">
        正文内容
    </div>
    <div style="width: 100px; position: absolute; top: 0px; bottom: 0; left: 0; z-index: 1; background-color: red; overflow: auto;">
        <div style="margin-top: 100px;">
            正文内容没设置背景色，所以这块内容没有被遮住
        </div>
    </div>
</div>