.slider {
  max-width: 400px;
  margin: 20px auto;
  text-align: center;
}
.slider__input {
  width: 100%;
}
.slider__image {
  display: block;
  margin: 20px auto;
  max-width: 100%;
  transition: width 0.2s ease;
}
#box {
  width: 50px;
  height: 50px;
  background-color: plum;
  position: fixed;
  pointer-events: none;
  transform: translate(-50%, -50%);
}