Ahh. It's not actually on my system: the same thing happens both on my phone thru WiFi at home, or on my PC. Based on the URL of frontier.com, my ISP is apparently "helping me" by injecting their search when I try to go to a domain that doesn't exist -- probably because I'm using their DNS.
Aah, I see now. Maybe I can change that to a different, non-existing domain that does not get served by them. Maybe using localhost with a port number that is hopefully unused will work better, or have a different failure mode at least ...