vue2.0上做切换过渡时，发现切换显示正常，切换消失时有延时没有过渡效果

<transition name='fade'>
        <p v-show="isShow" class="detail" >
            
        </p>
        </transition>

.detail
            position fixed
            z-index 100
            top 0
            left 0
            width 100%
            height 100%
            overflow auto
            background rgba(7,17,27,.8)
            &.fade-enter-active,&.fade-leave-active
                transition opacity 3s
            &.fade-enter,&fade-leave-active
                opacity 0