Reading about Quartz on the Mac I became super interested in Quartz Compositor and how the Mac renders windows. The linked article says the Mac is a able to render windows with holes whereas Windows (at least at the time) couldn't do that.

I know nothing about this—but I'm curious to explore this concept.

Is there an Open Source library similar to Quartz Compositor to learn from?

Good article on Quartz Compositor: http://web.archive.org/web/20100609024323/http://macdevcenter.com/pub/a/mac/2005/10/11/what-is-quartz.html?page=3

Only result on Github that references Quartz Compositor directly: https://github.com/jcampbell05/OpenCompositor