Thanks for your reply. The same hour you were responding I had given up and begun re-installation--but this time with Sabayon 10. So far I am not experiencing the login screen issue.
To benefit anyone going through this, here are some more details.
My laptop is a System 76 Gazelle Pro (gazp6) with the following video card:
Nvidia GeForce GTX 560M Graphics with 1.5GB GDDR5 Video Memory card.
Before I installed Sabayon 10, I re-installed Sabayon 9 and followed the post-installation steps here:https://wiki.sabayon.org/index.php?titl ... figuration
and this reproduced the bug. My installation of some nvidia-drivers in the first instance seems unrelated in retrospect. Unfortunately, I did not capture the log files before re-installation. However, if it happens again I will do so.