gitlab-org--gitlab-foss/app/assets/stylesheets/framework/vue_transitions.scss

.fade-enter-active,
.fade-leave-active,
.fade-in-enter-active,
.fade-out-leave-active {
  transition: opacity $sidebar-transition-duration $general-hover-transition-curve;
}

.fade-enter,
.fade-in-enter,
.fade-out-leave-to,
.fade-leave-to {
  opacity: 0;
}