This should have been fixed by Bug 495946 which introduced the `mailnews.sort_threads_by_root` preference to configure this behavior, see [Searchfox](https://searchfox.org/comm-central/rev/f976c25b5a228846cb59d3786fecb11eb4da1629/mailnews/mailnews.js#101-104). However, that patch missed to prevent threads from being resorted if they receive a new message (depending on the setting of that preference). This should be easy fix though. (Maybe right away with bug 1649402).
Bug 383985 Comment 24 Edit History
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This should have been fixed by Bug 495946 which introduced the `mailnews.sort_threads_by_root` preference to configure this behavior, see [Searchfox](https://searchfox.org/comm-central/rev/f976c25b5a228846cb59d3786fecb11eb4da1629/mailnews/mailnews.js#101-104). However, that patch missed to prevent threads from being resorted if they receive a new message (depending on the setting of that preference). This should be easy to fix though. (Maybe right away with bug 1649402).