Resolved. The symptoms, anyway. One setting change in Look and Feel did the trick. I turned off "Show splash screen at startup." All symptoms (of whatever the actual causative problem is) ceased existing. No subsequent row-size discrepancies from having Find Duplicates installed, changing book details, changing version, or changing font. Then I changed "Extra spacing to add between rows in the booklist" from 4px to none, to get back to the narrower row spacing I'm used to, which (also tested) didn't affect anything adversely.
This configuration (in Preferences > Look and Feel > Main Interface) seems stable on my system:
User interface style: calibre
User interface layout: narrow
Language: English
Checkboxes: all unchecked except Show tooltips
Interface font: Lucida Grande regular 13
Icon size: small
Show text under icons: never
Extra spacing between rows: none
Now I can stop chewing that bone.