amis/scss/components/_collapse.scss

.#{$ns}Collapse {
    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 .35s ease;
        &.in, &.out {
            height: 0;
            overflow: hidden;
        }
    }
}