Problem solved by L&L support. I needed to select the Scrivener.exe file, right-click for the Properties dialog, and set “compatibility mode” to Windows 7. I run a lot of new software on this system, and haven’t needed that old trick for many years, so I neglected to try it among my other Scriv3 troubleshooting efforts. (My next PC will be Windows 10, but only because I’ll have no choice. I am not looking forward to the downgrade.)
Anyway, happy ending. Now I just have to wrap my brain around all the changes in Scriv3…