Last time this showed up:
https://news.ycombinator.com/item?id=12830206
And for anyone else using Emacs on a Mac:
1. I use the plain vanilla GNU Emacs from here: https://emacsformacosx.com/. It does flicker from time to time, but it doesn't bother me.
2. Some Mac users seem to prefer this: https://www.emacswiki.org/emacs/EmacsForMacOS It's supposed to be flicker-free, among other things, but doesn't play well with MacOS native fullscreen (which I like), at least the last time I tried it.
On Mac I use the emacs-mac port by Mitsuharu Yamamoto: https://bitbucket.org/mituharu/emacs-mac/overview
which, in my opinion, works well. It can also be installed via brew (from source) or brew cask: https://github.com/railwaycat/homebrew-emacsmacport