fix ios scroll head

@@ -173,7 +173,7 @@ export default {
     },
     onDounceScroll({item, scrollTop, startIndex}) {
       this.scrollTop = scrollTop;
-      if (scrollTop === 0) {
+      if (scrollTop <= 0) {
         this.showHeader = false;
       } else if (item) {
         if (this.currentId !== item.data.articleId) {