Dropdown box remains highlighted after an item is selected

Quite a simple problem. I have a simple top menu which uses a dropdown box to select a sub-menu option from. After clicking on the option, however, the dropdown box remains in its highlighted state. If you click anywhere else in the game (not just on another button) the highlight disappears, but it nevertheless looks silly.

I know people frequently run into a similar problem when using normal buttons and they solve this by setting Navigation to None but unfortunately that does not work here.

Any help would be appreciated