zammad/app/frontend/apps/desktop/styles/transitions.css

.fade {
  &-enter-active,
  &-leave-active {
    transition: opacity 0.3s ease;
  }

  &-enter-from,
  &-leave-to {
    opacity: 0;
  }
}

.fade-move {
  &-enter-active,
  &-move,
  &-leave-active {
    transition: opacity 0.5s ease;
  }

  &-leave-active {
    position: absolute;
    top: -100%; /*  Add overflow clip on parent to hide elements */
  }

  &-enter-from,
  &-leave-to {
    opacity: 0;
  }
}

.collapse {
  &-enter-active,
  &-leave-active {
    transition:
      height 0.2s ease-in-out,
      opacity 0.2s ease;
    overflow: hidden;
  }

  &-enter-from,
  &-leave-to {
    opacity: 0;
  }
}