I've done some more looking and experiments using 2 tb clients accessing the same mailbox and to me it appears that if you are in "move to trash" delete mode and you have at least 20 messages in the folder that are marked \deleted, when that folder is selected it will be expunged. So it is unlikely that you will ever build up a huge number of \deleted messages in a folder. In addition, if a folder is selected when you shut down tb, the selected folder(s) will be expunged when in "move to trash" mode because imap CLOSE command occurs. Only if you are in "just mark as deleted" mode will the folder never be expunged. This behavior is with the several pref items containing the work "expunge" at their default settings in config. editor. This is handled by this piece of code: https://searchfox.org/comm-central/rev/631bd8e8e555d57fb21c3fb18f388516116549d9/mailnews/imap/src/nsImapProtocol.cpp#4201 So displaying messages that are deleted seems pointless since there shouldn't be many of them (unless you operate in "just mark as deleted" mode, which does show the deleted messages crossed out).
Bug 1644130 Comment 11 Edit History
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I've done some more looking and experiments using 2 tb clients accessing the same mailbox and to me it appears that if you are in "move to trash" delete mode and you have at least 20 messages in the folder that are marked \deleted, when that folder is selected it will be expunged. So it is unlikely that you will ever build up a huge number of \deleted messages in a folder. In addition, if a folder is selected when you shut down tb, the selected folder(s) will be expunged when in "move to trash" mode because imap CLOSE command occurs. Only if you are in "just mark as deleted" mode will the folder never be expunged. This behavior is with the several pref items containing the word "expunge" at their default settings in config. editor. This is handled by this piece of code: https://searchfox.org/comm-central/rev/631bd8e8e555d57fb21c3fb18f388516116549d9/mailnews/imap/src/nsImapProtocol.cpp#4201 So displaying messages that are deleted seems pointless since there shouldn't be many of them (unless you operate in "just mark as deleted" mode, which does show the deleted messages crossed out).