Copied from /r/pico8: https://www.reddit.com/r/pico8/comments/g45cpk/interface_shimmer_on_macos/

> Is anybody experiencing this? It was blinking for a while, restart did not help, then I got distracted with something and now it's okay again.
>
> https://imgur.com/a/5vsVxdv
>
> Update: Then it shimmers again. It is just random. And it’s very annoying, as there is no way to fix it.

I've only just downloaded the latest version of Pico-8 having bought a new mac recently and yes I get the same issue. It does seem to be very intermittent, and is the same on the macbook pro screen, or on an attached screen.
Glad it's not just me experiencing it - thought I was going mad :-) (or had a dodgy cable)

I saw that briefly as well, yes. I almost reported it but then it went away after restarting (either the app or the machine I forget which) and I haven't seen it since. I cannot reproduce it now. (I'm currently at 0.2.0i, macOS 10.15.5. I don't remember which version of PICO-8 had the issue.)

I've only ever used Pico-8 on MacOS Catalina and have logged something like 200 hours without seeing anything like that. But apparently it's affecting some users.