Add $imap_poll_timeout to allow mailbox polling to time out.
Enable the polling flag for the NOOP in imap_check_mailbox(), the STATUS command in imap_buffy_check(), and the LOGOUT command. This is not intended to handle all blocking-IO related issues. However, the periodic NOOP and STATUS are the most frequent places for mutt to freeze up, especially after a laptop is sleep/woken. Since these are quick operations with little data, this is a good place to check if the connection is still working before hanging on a read.
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