Does the same thing happen in safe/troubleshoot mode (hold Shift when launching TB)?

Thank you for your prompt response. Yes, I just tried it in safe mode and it happened. It doesn't happen every time I delete an email, mostly just the first time after opening Thunderbird, but occasionally at other seemingly random times as well.

I don't know if this is a graphics driver issue or possibly one caused by an antivirus app, if it is scanning the TB profile folder. If you run Windows in safe mode, it will bypass startup apps like AV and apply basic drivers.

This happens here all the time when deleting a message triggers automatic folder compacting - there's a notification in the status bar. It happens more often if you set a low threshold for automatic compacting. Setting a very large threshold to effectively disable compacting isn't recommended either, as compacting is necessary maintenance. https://support.mozilla.org/en-US/kb/compacting-folders

The threshold is set to 1 MB here, and that works for me. Although the behavior is not pretty, it's not something I'd lose sleep about.

Thanks again. The issue is indeed a result of the compacting process. The compacting process takes about 3-4 minutes, rendering Thunderbird unusable for that period. I've been using Thunderbird for almost a decade, accumulating many thousands of messages, which I assume accounts for the time it now takes to compact, as it's become an issue only over the past year or so. Anyway, I'm relieved to know the cause.

The compacting process takes about 3-4 minutes

This sounds like a lot. Compacting primarily happens for folders where messages get deleted on a regular basis, e.g. Inbox. Are you accumulating a lot of messages in Inbox? This is usually a bad idea. Best practice is to keep Inbox as tidy as possible.

If compacting is happening too frequently or not completing promptly, it's often worthwhile to rebuild the folder by deleting the mbox and msf files from the profile, and letting TB create new files automatically when it's restarted:

http://kb.mozillazine.org/Compacting_folders#Real_fix