Try this as your full CSS:
@media (min-width: 769px) {
#site-navigation {
position: fixed;
left: 0;
right: auto;
top: 0;
height: 100%;
z-index: 9999;
transition: transform 0s ease;
transform: translateX(0);
}
.slide-opened #site-navigation {
transform: translateX(270px);
z-index: 8888;
transition: transform 300ms ease;
}
}