CSS Trick

back-top

html,body{
  scroll-behavior: smooth;
}
.back{
  position: sticky;
  float: right;
  top: -110px;
  margin-top: -50px;
  border-radius: 50%;
  background: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 320 512'%3E %3Cpath fill='%23ffffff' d='M177 159.7l136 136c9.4 9.4 9.4 24.6 0 33.9l-22.6 22.6c-9.4 9.4-24.6 9.4-33.9 0L160 255.9l-96.4 96.4c-9.4 9.4-24.6 9.4-33.9 0L7 329.7c-9.4-9.4-9.4-24.6 0-33.9l136-136c9.4-9.5 24.6-9.5 34-.1z'%3E%3C/path%3E %3C/svg%3E") center no-repeat dodgerblue;
  background-size: 50%;
  width: 50px;
  height: 50px;
  transform: translateY(calc(100vh + 50px));
}