Okay, I think finally found the actual cause: [This line](https://searchfox.org/comm-central/rev/b9a42fb9f9d7d9be2ee6adea36656d7088ac55fb/mailnews/base/src/FolderCompactor.cpp#223) in the `FolderCompactor` code also closes the databases of all subfolders of the one folder that's actually to be compacted. This closes the view of any subfolder currently opened. __STR:__ 1. Open a folder that has subfolders and delete a message. 2. Select a subfolder of that folder. 3. Compact the parent folder.
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Okay, I think I finally found the actual cause: [This line](https://searchfox.org/comm-central/rev/b9a42fb9f9d7d9be2ee6adea36656d7088ac55fb/mailnews/base/src/FolderCompactor.cpp#223) in the `FolderCompactor` code also closes the databases of all subfolders of the one folder that's actually to be compacted. This closes the view of any subfolder currently opened. __STR:__ 1. Open a folder that has subfolders and delete a message. 2. Select a subfolder of that folder. 3. Compact the parent folder.