I tried to switch from libinput to synapatics by installing the latter using apt from the standard repository. It seemed to work ok at first, although the parameters settings were not working well on my Z990-R-AAS9U1 laptop. So I changed a few (FingerLow, FingerHigh, etc.), which seemed to help. But when I copied the synaptics default conf file from /usr/share/xorg.conf.d to /etc/X11/xorg.conf.d, added those options to it, and rebooted, gala didn't start up. Fortunately I could login to a shell and move 60-synaptics.conf out of /etc/X11/xorg.conf.d, and then I was able to get back to my usual desktop.
Anyone have any ideas why this disaster happened? And how to fix it?