documentation/.vuepress/theme/DropdownTransition.vue

<template>
    <transition name="dropdown"
                @enter="setHeight"
                @after-enter="unsetHeight"
                @before-leave="setHeight">
        <slot></slot>
    </transition>
</template>

<script>
    export default {
        name: 'DropdownTransition',
        methods: {
            setHeight(items) {
                // explicitly set height so that it can be transitioned
                items.style.height = items.scrollHeight + 'px';
            },
            unsetHeight(items) {
                items.style.height = '';
            }
        }
    };
</script>

<style>
    .dropdown-enter, .dropdown-leave-to {
        height: 0 !important;
    }
</style>