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Thunderbird 78.10.2 - a number of local folders will not open - showing only default "Local Folders" pane

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My Thunderbird 78.10.2 now has a number of local folders which will not open. They show only default "Local Folders" pane. See image below - note that the folder "professional" is highlighted is the left panel and the adjacent column shows 33054 emails but that the right hand panel shows only the default which normally appears only when the parent "Local Folders" folder is selected (and thus highlighted). I can't be certain but this may have occured due to a mistaken copy of another folder to be a subfolder to the folder "professional". That folder also now shows only the default panel as above.

My Thunderbird 78.10.2 now has a number of local folders which will not open. They show only default "Local Folders" pane. See image below - note that the folder "professional" is highlighted is the left panel and the adjacent column shows 33054 emails but that the right hand panel shows only the default which normally appears only when the parent "Local Folders" folder is selected (and thus highlighted). I can't be certain but this may have occured due to a mistaken copy of another folder to be a subfolder to the folder "professional". That folder also now shows only the default panel as above.
Attached screenshots

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The problem was a very prolonged "building summary file" for the compromised folders. My solution was - - turn off network so T'Bird cannot receive new emails - go away for several hours until "building summary file" completes for each compromised folder - create a number of new folders and move emails from each compromised folder to these new folders. Keep the new folders small - once each compromised folder is empty, delete it Problem solved.

Plea to the Mozilla team - detect user actions which may result in very prolonged "building summary file" events and prompt user with warning eg "This action may result in a very prolonged rebuild of the summary file for the folder <folder name> which will substantially slow Thunderbird until it completes. Do you wish to go ahead?" with a Yes/No option.

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Valgt løsning

The problem was a very prolonged "building summary file" for the compromised folders. My solution was - - turn off network so T'Bird cannot receive new emails - go away for several hours until "building summary file" completes for each compromised folder - create a number of new folders and move emails from each compromised folder to these new folders. Keep the new folders small - once each compromised folder is empty, delete it Problem solved.

Plea to the Mozilla team - detect user actions which may result in very prolonged "building summary file" events and prompt user with warning eg "This action may result in a very prolonged rebuild of the summary file for the folder <folder name> which will substantially slow Thunderbird until it completes. Do you wish to go ahead?" with a Yes/No option.