message: [improvement] DropdowmMenu: optimize rect calc
diff: @@ -44,7 +44,7 @@ export default sfc({
  toggleItem(active) {
  const { menu } = this.$refs;
  const rect = menu.getBoundingClientRect();
  - this.top = rect.top + rect.height;
  + this.top = rect.bottom;
  this.children.forEach((item, index) => {
  if (index === active) {
  