Edit in JSFiddle

<div id="header">
    <div>aaa</div>
    
    <div id="navi">absolute</div>
</div>
<div id="container">
    <div>bbb</div>
</div>

#header,
#container {
    position:relative;
    height:100px;
}
#header {
    background:#4169e1;
}
#container {
    background:#ffff00;
}
#navi {
    position:absolute;
    top:0px;
    left:100px;
    background:#ff4500;
    height:300px;
    width:100px;
    
    /* Important! */
    z-index:2;
}