I found references to xinput list
and that showed me the ETPS/2 Elantech Touchpad at id=12. Running
while true ; do xinput query-state 12 ; done
I noticed that button[1]=down remained down for the annoying 250ms, so something new in Ubuntu 20 is setting that - it didn't happen under 16.
I found the ETPS/2 Elantech Touchpad documentation here: https://www.kernel.org/doc/html/v4.14/input/devices/elantech.html
but I don't know how to find the parameters[0] values mentioned in that document.
My /sys/bus/serio/drivers/psmouse/serio2/ has reg_07 to 22, so I tried setting register 22 but it didn't seem to affect anything.
Any ideas for where else to look?
Thanks