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-dialogv-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>