Aah, now I understand.

There is no such setting that I'm immediately aware of - but I am automatically getting messages for my folders. So I'm assuming it's doing something in the background: most likely a periodic sync for the entire account.

Definitely doesn't sound like the behavior you wanted, my bad. But, at least there are notifications on the folders once mail is received. 😅

Edit: In the repo for their sync engine it explains:

Mailspring uses a fairly basic syncing algorithm, which runs on two threads with two open connections to the mail server. Within each thread, work is performed synchronously.

Background Worker: Periodically iterates over folders and (depending on the supported IMAP features) uses CONDSTORE / XYZRESYNC to check for mail or performs either a "local" or "deep" sync of part of the folder's UID space.

Foreground Worker: Idles on the primary folder and wakes to syncs changes. Also wakes to perform other tasks, like fetching message bodies the user clicks.

So the foreground worker only idles on the primary folder, but it does slowly iterate over all folders in the background.

No settings have been added for this functionality (that I'm aware of).

Edit 2: Went back and read your original comment; hadn't noticed the PR was yours. If that's the case then you're probably aware of how the sync works anyway. My bad if I've posted stuff you're already aware of.