Note: I have one open that I did not click to bring in focus and the Windows 10 preview shows the subject line from an email either 4 or 5 more recently in sequence received; the body is unchanged in the preview--until/unless I click it into focus.

Same email, still not clicked on, and subject line changed again to one 6 high in the list after I archived old emails and "repaired" the folder.

I suggest you try using tags to mark messages for action rather than reply on open windows or tabs. Windows and tabs have a long history of having odd things happen or just all being closed on a restart.

Essentially Thunderbird keeps track of message based on their relative location in a file. When a compact occurs( essential by the way) that location can and often does change. While attempts are made to not loose track of open windows, as you have observer it is not 100%.

Tags on the other hand are fairly robust. Especially if you use the default 9 categories. It might be a slight change of pace, but as you can also save a search for tagged messages as a virtual folder, your "todo" list can actually appear as a folder in the folder pane.

I do use tags for this purpose. But I prefer the open windows (as it's more of a nag to me and does not have me filtering my emails to the tags all the time). It was working (except when Thunderbird crashed on me at times, and then I lost them) until recently. Maybe compaction settings/timings changed?

NEW WRINKLE: I just hit reply on an email which displayed just fine (correct subject line in the Windows title bar and correct body copy)--but the reply it generated was going to a different email address and subject (and was above a different body) that was two emails more recently received. THIS IS A SERIOUS BUG!

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