Plugins which wrap the selectmenu content make it non-clickable

e.g. when using the jScrollPane plugin for a custom scrollbar inside the menu dropdown like this:

``` js
$('select').selectmenu().next('span').find('a.ui-selectmenu').one('click', function() {
    $('ul.ui-selectmenu-menu-dropdown:visible').jScrollPane({
        isScrollableH: false
    });
});
```
