Test at gmail site, setting label on message in folder [Gmail]/f1 to have label [Gmail]/f3 also results in redundant storage in the tb mbox file (the full message appears in files f1 and f3. So now I don't know if this is a bug or not. I wasn't around when gmail was added to tb so I don't know the history firsthand. I have seen maybe more than one "meta" bugs regarding gmail support such as this: Bug 402793 (tb-gmailWIP). Looking at one of the bugs listed, Bug 901287, it seems to claim that the feature is present (Bug 901287 comment 2) but the bug is closed with resolution INCOMPLETE. So my guess is that this feature was attempted and something was added to tb but it never quite worked right.
Bug 1554529 Comment 6 Edit History
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Test at gmail site, setting label on message in folder [Gmail]/f1 to have label [Gmail]/f3 also results in redundant storage in the tb mbox file (the full message appears in files f1 and f3). So now I don't know if this is a bug or not. I wasn't around when gmail was added to tb so I don't know the history firsthand. I have seen maybe more than one "meta" bugs regarding gmail support such as this: Bug 402793 (tb-gmailWIP). Looking at one of the bugs listed, Bug 901287, it seems to claim that the feature is present (Bug 901287 comment 2) but the bug is closed with resolution INCOMPLETE. So my guess is that this feature was attempted and something was added to tb but it never quite worked right. Edit: I just realized that INCOMPLETE probably means the reporter never responded, not that the fix is incomplete.