I had just found out that a directory switch within vim was causing my problem right before I saw your message, and now I know what .vimrc setting was doing it to me: yes, vim was autochdir’ing to the spec file’s directory, and vim in turn was spawning a tmux runner pane, in a subdirectory where rspec didn’t know how to locate rails_helper.
Having turned off autochdir, things are working as expected (as long as I’m working in the Rails project root).
That was a bear of a morning. Thanks for your help. Nice course, too. I don’t know if I’ll spend 100% of my time in Tmux, but the number is definitely going way up.