Ok, it’s working now. I had to set a system setting via the command line (DisplayResolutionEnabled) in order to see the HiDPI selection on my Mac desktop computer, which allowed me to select a HiDPI mode on my monitor. Now I am getting the 2x images I am supposed to see.
(Unfortunately, my non-Apple hi-resolution monitor still doesn’t have a HiDPI selection available for this testing for some reason, but I can use my regular monitor to see that the correct images are being used, and that’s enough for my development and testing purposes.)
Thanks for the help, Arne!