body {
  display: flex;
  justify-content: center;
  align-items: center;

  height: 100vh;
  width: 100vw;

  background: linear-gradient(
    to bottom,
    #fff 0%,
    #fff 50%,
    #1e90ff 50%,
    #1e90ff 100%
  );
}
.container {
  --num: 0;
  --duration: 1s;
  --size: 100px;
  --transition-timing-function: cubic-bezier(0.68, -0.55, 0.265, 1.55);

  display: flex;

  height: var(--size);
  width: var(--size);

  background: #fff;
  border-radius: 3px;

  box-shadow: rgba(0, 0, 0, 0.24) 0px 3px 8px;
  overflow: hidden;

  font-family: Arial, "Helvetica Neue", Helvetica, sans-serif;

  transition: all var(--duration) var(--transition-timing-function) 60ms;
}
.container.active {
  width: calc(var(--size) * calc(var(--num) + 1));
}
.container .content {
  display: flex;
  flex-wrap: nowrap;
  opacity: 0;

  height: 100%;
  width: 0px;

  transition: all var(--duration) var(--transition-timing-function);
}
.container.active .content {
  opacity: 1;

  width: calc(var(--size) * var(--num));
}

.container .content .item {
  height: 100%;
  width: var(--size);

  flex-shrink: 0;

  line-height: var(--size);
  text-align: center;

  font-size: 1.2rem;

  transform: rotateY(450deg);

  transition: all var(--duration) var(--transition-timing-function);
}

.container.active .content .item {
  transform: rotateY(0deg);
}

.container .btn {
  display: flex;
  justify-content: center;
  align-items: center;

  height: var(--size);
  width: var(--size);

  z-index: 1;

  outline: none;
  border: none;

  background-color: transparent;
}

.container .btn .line {
  height: 0.3rem;
  width: 1.5rem;

  background-color: #1e90ff;

  border-radius: 0.3rem;
  transition: all var(--duration) var(--transition-timing-function);
}
.container .btn .line:nth-child(1) {
  transform: translate(0.75rem, -0.5rem);
}
.container.active .btn .line:nth-child(1) {
  transform: translate(0.75rem, 0rem) rotate(405deg);
}
.container .btn .line:nth-child(2) {
  transform: translate(-0.75rem, 0.5rem);
}
.container.active .btn .line:nth-child(2) {
  transform: translate(-0.75rem, 0rem) rotate(-405deg);
}