The menu is wider than the widget, and the right border is broken.
Providing a good UI while enabling old and mobile browsers sometimes results in compromises. There are dropdown menus as native OS components, which are automatically wider than the corresponding calling elements. We're not hindering its functionality, provide widest browser support possible and it's limited to PHP (non-infused) widgets only.
Therefore closing as declined.
Just tweaking the padding and image position makes it look better in Chrome & Firefox:
(set width to 100%+1px, and background position to right 1px)
which represents a vast majority of our users.
In which browsers would this technique not work?