I’m sharing my laptop touchpad PROBLEM+SOLUTION in case it’s of help to anyone else using an old MacBookPro laptop for Manjaro/Arch experiencing similar problems…
SYMPTOM: Screen cursor bouncing all over the place, selecting, clicking, editing my desktop (!) and generally behaving like a rabid input device running amuck. It would begin after the laptop had been on for a while and nothing I could do would stop it … except poweroff via a hard power button crash.
I shut down and rebooted repeatedly - detaching various USB connections - trying to figure it out. Finally, reading about how old MBP laptop owners were experiencing this within OSX, I concluded that it was the flailing gasps of my laptop’s aging touchpad.
It’s hard to prove a negative, but that crazy manic cursor behavior has not occurred again (since).
ENDNOTE: If it turns out the problem reoccurs - even with the touchpad kept disabled - then it will show that my diagnosis is incorrect and I’ll share an update accordingly.