Thanks! I renamed both .gimp-2.8 and the .config/GIMP directories and gimp started without any remembered settings, as expected, but the problem remains.
I mimicked your desktop settings/colors/etc and the problem remains... No combination of desktop settings seems to make the drop-down menus visible. (I'm using Breeze (or Breeze Dark) Look and Feel and Desktop Themes, Colors are set to Breeze Dark, but it doesn't matter which I choose, the issue persists across all of them.)
I tried your portable version (after also renaming .gimp-2.8 and .config/GIMP) and get the same results.
To be clear, though, I'm not talking about the application menus (File, Edit, Select, etc) I'm talking about what I'm calling "drop-down menus" (e.g. in
the image I posted above, the "Template" setting) e.g. when making a new file, or the overlay modes in the layers palette, etc.
I just realized that I was getting these errors on the CLI when starting GIMP:
(gimp-2.9:19253): Gtk-WARNING **: Unable to locate theme engine in module_path: "adwaita",
(gimp-2.9:19253): Gtk-WARNING **: Unable to locate theme engine in module_path: "adwaita",
(gimp-2.9:19253): LibGimpBase-WARNING **: gimp-2.9: gimp_wire_read(): error
...so, following a forum thread I found, I installed the gnome-themes-standard package, and all those errors are now gone, but... the problem remains. :-)
I am still seeing "gimp_display_shell_profile_update" three times on the CLI, in case that means anything.
Anything else to try? Thanks for help!