Edit in JSFiddle

.mainpane {
    position:relative;
     overflow-x: auto;
     overflow-y: auto;
}
.timeline {
    background-color:lightgray;
    border:solid 1px black;
    position:relative;
    

}
.ribbon {
    float: left;
    margin:2px;
    width:64px;
}
.event {
    background-color:white;
    border:solid 1px black;
    position:absolute;
    margin:2px;
    box-shadow: 10px 10px 5px #888888;
}