[Fixes #15944] Add Z-Index to avoid border overlapping

Created by: kkirsche

[Fixes #15944 (closed)] Add Z-Index to avoid border overlapping

less/input-groups.less

@@ -161,6 +161,7 @@
     > .btn,
     > .btn-group {
       margin-left: -1px;
+      z-index: 2;
     }
   }
 }