the-modal.css

.lock {
    /* when modal is opened we're removing scrollbars from the main content */
    overflow: hidden;
}

.shim {
    /* shim will stay fixed and will take all the space available */
    position: fixed;
    bottom: 0; left: 0; top: 0; right: 0;

    /* if modal content doesn't fit inside the shim, display scrollbars */
    overflow: auto;

    /* allow one-finger iPad scrolling */
    -webkit-overflow-scrolling: touch;
}

/* fix for iPad glitches */
.shim > * {
    -webkit-transform: translateZ(0px);
}

.shim {
    background: rgba(0,0,0,0.5);

    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr=#7F000000,endColorstr=#7F000000); /* IE6–IE8 */
    zoom: 1;
}