"By default, SQLite will automatically checkpoint whenever a COMMIT occurs that causes the WAL file to be 1000 pages or more in size, or when the last database connection on a database file closes. "
Thank you jademan.
It is probably the close that is triggering the checkpoint call, and in my large 45-minute tracks I see the popup; probably the 00:03:42 music tracks from which I have trimmed front and back of applause (evil grin) have too little housekeeping for the pop-up to get out of bed.
Hence I see it at the end of an audio-book track, but never (as I recall) at the end of any minor task.
I am not harmed by the popup delay - mostly I see it as I push back from the desk to get a fresh mug of tea - but I was puzzled by its apparent random appearance.
Cheers
Chris