trailbase/trailbase-core/js/shared/kobalte.css

.collapsible__content {
  overflow: hidden;
  animation: slideUp 300ms ease-out;
}
.collapsible__content[data-expanded] {
  animation: slideDown 300ms ease-out;
}

@keyframes slideDown {
  from {
    height: 0;
  }
  to {
    height: var(--kb-collapsible-content-height);
  }
}
@keyframes slideUp {
  from {
    height: var(--kb-collapsible-content-height);
  }
  to {
    height: 0;
  }
}