I struggled to get a projector working with macOS. This was entirely unexpected since I’d never had issues in the past. It always “just worked” in usual Apple fashion. I ran out of time and had to give up.

It continued to nag me, so I started digging further. And then the obvious problem hit me. I’m using Cody Krieger’s excellent gfxCardStatus utility. I had it set to force the use of the Intel GPU in order to conserve battery power. And that was the problem. macOS requires the NVIDIA chipset in order to drive an external display. macOS won’t use the Intel GPU to do that.

Setting gfxCardStatus to automatically switch got things working again.