On Windows, the only key I found that closed the completion box via keyrelease_event is Left Arrow.  If I explicitly caught that, the method might not be needed on Windows.  Next I will look at the triplets list and how something might be missing or deleted twice.

On my Mac Airbook with current 3.8.6 and 3.9.0rc2, I cannot get a completion box, even with the menu.  Do you have the same issue?

Reports with a traceback, like this one, are helpful and welcome.  If possible, include and information about either the code or action that triggered the exception.