This is a tricky timing bug triggered by Gmail sending EXPUNGE and
EXISTS notifications in a particular order with a slight timing delay
between them.

Normally Gmail will send one or more EXPUNGE notifications followed by
EXISTS to finish up a list of removal notifications.  Even if the
client sends an APPEND followed by STORE/EXPUNGE, the EXPUNGE
notifications are first.

However, if the client STORE/EXPUNGEs on one connection and a new
message arrives from another (or via SMTP), Gmail will issue an
EXISTS (append), wait 250 msec, then issue the EXPUNGE(s).  In
that 250msec delay, Geary has already turned around and requested
the new message, which no longer exists at the reported position
because the EXPUNGEs (which have not arrived yet) have shifted it
downward.

The solution is to pause when notifications come in so all can
be accounted for, shifting positions as necessary.