I discovered the hard way that after the upgrade to nvidia-drivers-100.14.09,
in order for X to work, I had to
modprobe -r nvidia
by hand. Now, since the emerge process did not mention this, I naively
expected the driver to take care of it (well, to be honest, I had forgotten
about the kernel module).
It would be helpful if among all the messages at the end of the install there
was one to the effect:
"Don't forget to unload any old, incompatible nvidia kernel modules before you
use this version"
to remind people like me who do not keep track of things very well.