amis/scss/components/_collapse.scss

.#{$ns}Collapse {
  border: none;
  padding: 0;
  margin-bottom: $Form-item-gap;

  &-header {
    font-size: $fontSizeMd;
    font-weight: $fontWeightNormal;
    color: $text--loud-color;
    padding: px2rem(5px) 0;
    border-bottom: $borderWidth solid lighten($borderColor, 5%);
  }

  &-arrow {
    display: inline-block;
    width: px2rem(16px);
    text-align: center;
    margin-left: $gap-xs;
    cursor: pointer;

    &:before {
      content: '';
      position: relative;
      display: inline-block;
      width: px2rem(6px);
      height: px2rem(6px);
      top: px2rem(-4px);
      border-color: $text-color;
      border-style: solid;
      border-width: px2rem(1px) px2rem(1px) 0 0;
      transform: rotate(135deg);
      transform-origin: 50% 50%;
    }
  }

  &.is-collapsed &-arrow:before {
    transform: rotate(45deg);
    transform-origin: 0% 50%;
  }

  &--collapsable &-header {
    user-select: none;
  }

  &-content {
    transition: height 0.35s ease;

    &.in,
    &.out {
      height: 0;
      overflow: hidden;
    }
  }
}