Thanks for the bugzilla reference. I understood there was something wrong, and it’s good to know it’s been posted to bugzilla.
For background, not knowing about nomodeset workaround I had very successfully upgraded my existing system via the system-migration-tool, but didn’t like tiny custom login that it installed .
So I tried lightdm and then found myself in all sort of difficulties getting back to gdm (for screen lock).
But I guess I had better do a search and maybe if necessary post a question
Thanks @malcolmlewis @hui for your input 