默认为 15vh，当弹窗过于高的时候，这个距离其实是不合适的

<el-dialog
    v-model="dialogVisible"
    title="Tips"
    width="30%"
    :before-close="handleClose"
    top="6vh"
  >
    <span>This is a message</span>
    <template #footer>
      <span class="dialog-footer">
        <el-button @click="dialogVisible = false">Cancel</el-button>
        <el-button type="primary" @click="dialogVisible = false">
          Confirm
        </el-button>
      </span>
    </template>
  </el-dialog>