How to reproduce:
Let's say you have +30 results on something and want to limit the height of your autocomplete to maintain a good UI

If the user navigates through the result list, the autocomplete should scroll when he selects a Textcomplete-option outside of the viewport.
There was a similar issue with the jquery ui autocomplete back in the days.
http://stackoverflow.com/questions/7045858/jquery-ui-autocomplete-not-scrolling-with-arrow-keys-in-firefox
Maybe that helps with the fix.