The physical process of compacting requires that the disk has enough free space that the folder can be duplicated. In practical terms the folders contents that is undeleted is copied to a new file and the old one deleted. Then the new folder is renamed to replace the old one.

I do not recall seeing any new bugs about compacting, so I am guessing that your issue is more than likely caused by an antivirus product scanning. This can fool Thunderbird into thinking there is a disk space issue as attempts to write a new file result in a failure as the antivirus has the relevant file locked for scanning.

Looking in the physical profile folder, folk with these sorts of issue routinely have many files in their profile folder with the same name and a number appended. So I would suggest you have a look before moving onto trying to fix it.

It is a known but benign condition that can be ignored for a few weeks until it is fixed. But you will need to update, and version 138 is rather old.

I finally got the notice of version update to 141. I did that and it seems to help. Thunderbird still updates often, but the sizes/time to compact are significantly less and the message list doesn't go blank while it happens. Guess I can turn on auto compact again. BTW it would be nice to be able to type in the minimum size needed before compacting instead of having to hold down count-up button forever.

Wish the update notices came sooner. Many thanks for the help.