User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:65.0) Gecko/20100101 Firefox/65.0

Steps to reproduce:

How to reproduce:

1. Copy a significant amount of mails into test folder (I tried this several times with 40 mails and couldn't reproduce - with 300 mails, I "succeeded" on my first try).

2. Select all mails.

3. Hit the "archive" button.

4. Hit the "archive" button AGAIN. Important: You need to do this before the progress bar starts at the bottom of the screen. If you do it once the progress bar has started, it will not lead to message loss but only to additional empty messages being created in the archive folder (which isn't nice either, but not my concern).

I am assuming that all this is dependent on the speed of your computer (mine is several years old). If your machine is very fast, you might have to do this with 3000 instead of 300 messages.

From what I am second-guessing, this seems like a serious design flaw. It appears like, before the progress bar starts, Thunderbird probably starts marking messages as deleted and, at the same time, makes a list of the files that should be copied. When you re-click the "Archive" button during this phase, Thunderbird keeps a comprehensive "to delete" list, but starts the "to archive" list from scratch, therefore omitting the messages it has processed so far.

From my testing, message destruction "works" both with local and IMAP folders.

Sounds a bit like Bug #757328 but I'm posting this as a fresh bug since I don't believe anyone would leave a serious, message-eating bug unfixed for 6+ years.

PS: I came across this problem last October, but have been barred by bugzilla from posting it. Hence, the "version numer" above is only an estimate. But given the fact that nobody has posted it in the meantime, I'm positive it is still around.

Seems to be a duplicate of bug 757328. Archiving isn't the best maintained area, so it's possible that "a serious, message-eating bug" hasn't been addressed.