Thanks for your reply, Joel. Yes, I've checked the firewall and router settings multiple times. I'm on a Mac, and Firewall is "off" and ipfw has no restrictions:
sh-3.2# sudo ipfw list
65535 allow ip from any to any
and port 8085 works fine, for the debugger. It's just 8080 that is causing trouble. I tried connecting both the computer and the Roku to a hotspot app on my Android tablet, which creates a separate subnet, and got the same behavior. I think it's the Roku, not my configuration, but I can't think of what else to try.