.back-to-top {
  position: fixed;
  z-index: 2;
  right: -108px;
  bottom: 0;
  width: 108px;
  height: 150px;
  background: url("../images/back-to-top.png") no-repeat 0 0;
  background-size: 108px 450px;
  opacity: 0.6;
  transition: opacity 0.3s, right 0.8s;
}
.back-to-top:hover {
  background-position: 0 -150px;
  opacity: 1;
}
.back-to-top.load {
  right: 0;
}
.back-to-top.ani-leave {
  background-position: 0 -150px;
  animation: ani-leave 390ms ease-in-out;
  animation-fill-mode: forwards;
}
.back-to-top.leaved {
  pointer-events: none;
  background: none;
  transition: none;
}
.back-to-top.ending {
  pointer-events: none;
}
.back-to-top.ending::after {
  opacity: 1;
}
.back-to-top::after {
  content: '';
  position: fixed;
  z-index: 2;
  right: 0;
  bottom: 0;
  width: 108px;
  height: 150px;
  background: url("../images/back-to-top.png") no-repeat 0 0;
  background-size: 108px 450px;
  background-position: 0 -300px;
  transition: opacity 0.3s;
  opacity: 0;
  pointer-events: none;
}