The upgrade that was happening seemed to complete fine. No packages were in a half-installed or unconfigured state, and every available update was completed. (And as I said, the update-manager UI didn't go away.)
It looks like gnome-settings daemon was missing, possibly crashed, at the time I saw the apport window. The g-s-d process wasn't running. I realized this when apport started firefox, the UI styling wasn't normal. I don't know if gnome-settings-daemon went down for reasons of its own, and triggered a problem in the update-manager UI. More on that if I think of anything.
More minor points -
The upgrade that was happening seemed to complete fine. No packages were in a half-installed or unconfigured state, and every available update was completed. (And as I said, the update-manager UI didn't go away.)
It looks like gnome-settings daemon was missing, possibly crashed, at the time I saw the apport window. The g-s-d process wasn't running. I realized this when apport started firefox, the UI styling wasn't normal. I don't know if gnome-settings- daemon went down for reasons of its own, and triggered a problem in the update-manager UI. More on that if I think of anything.