And that's going about as well as expected:
But the above game of whack-a-mole is not why I'm posting. No, I'm posting because I'd like someone to explain to me why my iMac's second monitor is working fine but my third monitor (my projector) is getting no HDMI signal. I see a bunch of people complaining about this sort of thing on laptops, but no answers.
If I go to the end of the run and plug in a different monitor into the cable that normally plugs into the projector, I get signal, main screen turn on. But apparently said signal is now unacceptable to my projector.
Note: no hardware has changed, only an OS update. And somehow that OS update has weakened or otherwise broken this HDMI run, and now I can no longer watch TV from my couch.
Double-You Tee Fuck.
Update: And then... it magically started working again.