Content assist suggestion window disappears on failure to match typeahead

[tedepstein]

If you invoke content assist and start typing, it filters the list until you type something that doesn't match whatever is in the list. Then the suggestion list disappears, and you can't get back except by backspacing and re-invoking content assist.

This behavior might be reasonable by itself. But in combination with case-sensitive matching as described in #268 , it makes content assist especially fragile and user-unfriendly. I suggest fixing #268 first, and then take another look at this to see if a more friendly behavior is possible/practical.