leaflet-side-by-side/layout.css

.leaflet-sbs-range {
    position: absolute;
    top: 50%;
    width: 100%;
    z-index: 999;
}
.leaflet-sbs-divider {
    position: absolute;
    top: 0;
    bottom: 0;
    left: 50%;
    margin-left: -2px;
    width: 4px;
    background-color: #fff;
    pointer-events: none;
    z-index: 999;
}