Added z-index property so that the drop down menu is top of the screen in edit mode.

Considering the full screen layer, added a margin.

resource/css/crowi.scss

@@ -190,6 +190,7 @@ footer {
 // }}}
 
 .dropdown-menu {
+   z-index: 10060;
   .dropdown-button {
     padding: 3px 20px;
   }