I find that the first time I open the quick-search bar after starting Sylpheed
(which is by default hidden, so I have more message view area), it opens
"upwards", meaning that the summaryview area is reduced, and the messageview
area remains the same.

Unfortunately, when closing, it closes "upwards" as well. That is to say, the
summaryview area remains the same, and messageview area expands.

The net result is that, after turning the quicksearch bar on and then off again,
the messageview area is larger than it was originally, and the summaryview area
is reduced. This only happens for the first open/close cycle; thereafter, it
opens "up" and closes "down", as is expected.

(Might I also suggest that it opens "down" and closes "up"? Since the
quicksearch bar just filters what's displayed in the summaryview, it makes sense
to leave the summaryview area unchanged and instead reduce the messageview area;
the summaryview area is probably what the user is going to be interested in
after opening up the quicksearch bar)

Excellent work, BTW :)