[juanferna]

Originally Posted by kovidgoyal

This should fix it:
https://github.com/kovidgoyal/calibr...c473f813b92099

Apparently the code to process EPUB files is now so fast the filesystem cant keep up with it.

Tested with Calibre 8.4, it still occasionally has issues.
However, 8.4's behavior is much better than 8.3's: in cases where it still doesn't work properly, the new popup appears and there's no need to restart the viewer, so I consider this a low-priority issue.
Additional information is attached.