如何确保vuetify的下拉菜单具有最高的z-index？

.custom-menu {
  z-index: 9999;
}

<template>
  <v-app>
    <v-menu class="custom-menu">
      <template v-slot:activator="{ on }">
        <v-btn v-on="on">Toggle Menu</v-btn>
      </template>
      <v-list>
        <v-list-item v-for="item in items" :key="item.title">
          <v-list-item-title>{{ item.title }}</v-list-item-title>
        </v-list-item>
      </v-list>
    </v-menu>
  </v-app>
</template>

<script>
export default {
  data() {
    return {
      items: [
        { title: 'Item 1' },
        { title: 'Item 2' },
        { title: 'Item 3' }
      ]
    };
  }
};
</script>

<style>
.custom-menu {
  z-index: 9999;
}
</style>