It’s appearing on desktop as you gave it a width of 65%: width: 65%;
To make it 65%, try this:
.offside {
width: 65%;
}
.offside,
.offside--left {
left: -65%;
}
.offside--left.is-open,
.offside-js--is-left .offside-sliding-element {
-webkit-transform: translate3d(100%,0,0);
-moz-transform: translate3d(100%,0,0);
-ms-transform: translate3d(100%,0,0);
-o-transform: translate3d(100%,0,0);
transform: translate3d(100%,0,0);
}
.offside-js--is-left.slide-opened .slideout-overlay button.slideout-exit {
left: 65%;
}