Edit: I messed up, see below
OK, I think I must be doing something wrong. I'm just rebooting in case...
I've found this afternoon that most of my older messages keep disappearing from the Smart Inbox. When I go into my main IMAP inbox, it seems to re-sync and download them again, and then I can see them until either something unspecified happens or a new email comes in, at which time it may revert to just showing the last few unread.
I've tried various difference settings for the Smart Inbox - well, either only unread or 1 week - and I'm not sure they make a difference. It seems more related to what AquaMail thinks is in my IMAP inbox.
I only have at most 6-8 unread and 2 starred emails in there, at the oldest from mid-November. It'd be nice to just have them kept there, and the Smart Inbox showing me whatever is there in my mailboxes. Is this possible?
THANKS!
ETA: My apologies, I've solved this (sortof).
Postbox has inherited from Thunderbird a problem with its own smart inbox ("All Accounts") with IMAP servers, where when you file messages, it clears them display, but does not actually expunge them. So my inbox had perhaps dozens of deleted but unexpunged emails in it.
This must have gone over AquaMail's threshold for storing mail, even though it too was not showing the deleted messages. Having expunged my inbox, the widget and mailbox are now working as expected.