Back-to-top在Bootstrap 4中不再有效

<button id="back-to-top-btn" class="btn btn-primary" title="Back to Top">
  <i class="fas fa-arrow-up"></i>
</button>

#back-to-top-btn {
  position: fixed;
  bottom: 20px;
  right: 20px;
  display: none;
}

window.addEventListener('scroll', function() {
  if (window.pageYOffset > 100) {
    document.getElementById('back-to-top-btn').style.display = 'block';
  } else {
    document.getElementById('back-to-top-btn').style.display = 'none';
  }
});

document.getElementById('back-to-top-btn').addEventListener('click', function() {
  window.scrollTo({ top: 0, behavior: 'smooth' });
});