message: [Improvement] SubmitBar: optimize render
diff: <template>
  <div :class="b()">
  - <div :class="b('tip')" v-show="tip || $slots.tip">
  + <div :class="b('tip')" v-if="tip || $slots.tip">
  {{ tip }}<slot name="tip" />
  </div>
  <div :class="b('bar')">
  