Earlier quoted context omitted.
macOS uses a PDF-like canvas as the core display engine^, upon which everything writes object layers that have refresh loops independent from the main canvas 'flatten layers into the display' loop. This is all inside WindowServer, and is why lets you select an entire window including the shadowed border, without picking up anything from above or beneath it, because Spacebar activates object selection mode. And we kno…
This comment here is what I come to HN for. I think that OS X was the first OS that did hardware compositing. How come that WindowServer consumes so much CPU instead of GPU?
I didn't closely check for "is it CPU+GPU or CPU only" when I looked at Activity Monitor yesterday. I could, but I suspect that isn't a stable answer year-over-year yet.