/*****************************************/
/*  Boutons de navigation verticale      */
/*****************************************/
#scrollButtons {
  position: fixed;
  right: 20px;
  top: 50%;
  transform: translateY(-50%);
  display: flex;
  flex-direction: column;
  gap: 10px;
  z-index: 9999;
}
#scrollButtons a {
  display: block;
  width: 40px;
  height: 40px;
  line-height: 40px;
  text-align: center;
  font-size: 20px;
  font-weight: bold;
  color: #fff;
  background: rgba(128, 0, 0, 0.6);
  border-radius: 5px;
  text-decoration: none;
  transition: background 0.3s ease;
  user-select: none;
}
#scrollButtons a:hover {
  background: rgba(128, 0, 0, 1);
}
#scrollButtons a:active {
  transform: scale(0.95);
}

<!-- Boutons de navigation verticale -->
<div id="scrollButtons">
  <a href="#" id="scrollTop" title="Haut de page">▲</a>
  <a href="#" id="scrollUp" title="Remonter d’un écran">▴</a>
  <a href="#" id="scrollDown" title="Descendre d’un écran">▾</a>
  <a href="#" id="scrollBottom" title="Bas de page">▼</a>
</div>
<script>
    const scrollTopBtn = document.getElementById('scrollTop');
    const scrollBottomBtn = document.getElementById('scrollBottom');
    const scrollUpBtn = document.getElementById('scrollUp');
    const scrollDownBtn = document.getElementById('scrollDown');

    scrollTopBtn.addEventListener('click', (e) => {
      e.preventDefault();
      window.scrollTo({ top: 0, behavior: 'smooth' });
    });
    scrollBottomBtn.addEventListener('click', (e) => {
      e.preventDefault();
      window.scrollTo({ top: document.body.scrollHeight, behavior: 'smooth' });
    });
    scrollUpBtn.addEventListener('click', (e) => {
      e.preventDefault();
      window.scrollBy({ top: -window.innerHeight, behavior: 'smooth' });
    });
    scrollDownBtn.addEventListener('click', (e) => {
      e.preventDefault();
      window.scrollBy({ top: window.innerHeight, behavior: 'smooth' });
    });
</script>