[bugfix] DropdownMenu: incorrect position in some browsers

docs/markdown/changelog.zh-CN.md

@@ -27,6 +27,7 @@
 ##### DropdownItem
 
 - 新增`title-class`属性
+- 修复在部分浏览器上定位错误的问题
 
 ##### Picker
 

packages/dropdown-menu/index.js

@@ -44,7 +44,7 @@ export default sfc({
     toggleItem(active) {
       const { menu } = this.$refs;
       const rect = menu.getBoundingClientRect();
-      this.top = rect.y + rect.height;
+      this.top = rect.top + rect.height;
 
       this.children.forEach((item, index) => {
         if (index === active) {