.back-to-top {
  position: fixed;
  right: 1.25rem;
  bottom: 1.25rem;
  z-index: 1000;
  width: 3rem;
  height: 3rem;
  display: flex;
  align-items: center;
  justify-content: center;
  border: 0;
  border-radius: 50%;
  background: #1a1a1a;
  color: #fff;
  font-size: 1.25rem;
  cursor: pointer;
  box-shadow: 0 4px 12px rgba(0, 0, 0, .2);
  /* état caché */
  opacity: 0;
  visibility: hidden;
  transform: translateY(1rem);
  transition: opacity .3s, transform .3s, visibility .3s, background-color .2s;
}

.back-to-top.is-visible {
  opacity: 1;
  visibility: visible;
  transform: none;
}

.back-to-top:hover {
  background: #333;
}

.back-to-top:focus-visible {
  outline: 3px solid #4d90fe;
  outline-offset: 3px;
}

@media (prefers-reduced-motion: reduce) {
  .back-to-top { transition: none; }
}