notmuch.el: controlling what does and doesn't get expanded in searches

[Matt Armstrong]

This question pertains to notmuch built from recent git HEAD, using
notmuch.el in show mode (i.e. not tree mode).

I sometimes read a thread with a bunch of messages and notmuch.el
collapses a bunch of them (even if unread and the search matches tags in
every message).  I can't figure out the heuristic notmuch is applying
here.  In particular, pressing SPC does not seem to navigate to the
collapsed messages (again, even if they are unread).

Worst case: only the first messages is initially expanded and all
subsequent are collapsed.  I press SPC and the cursor goes to the end of
search results.  SPC again all the entire thread is archived.

This behavior has caused me to accidentally skip messages.  First step
for me is understanding what is going on so I can fix it.