See also issue 1161031, especially Giampaolo's suggestion near the end.
 Seems like it might be relevant.  I was the one who merged Josiah's
trunk fix into othe other branches, to get things back into sync, but I
don't claim to understand the code at any deep level.  From the review I
did at my level of knowledge it seems like the change to the behavior of
when handle_expt_event is called is correct, but I was a little worried
about backward compatibility.  I now wonder if that fix should be backed
out of 2.6 and 3.0, and advertised in What's New for 2.7 and 3.1.