I was as well - everytime I see one of these they also pause too briefly at the change over from inhale/exhale and back.

It's disconcerting because its not how I naturally breathe.

Yes! In my periphery it looked like a polygon but when I focused on the edges it seemed like a properly antialiased circle.

Indeed. I kept wondering if the circle was actually getting lighter as it got smaller or if that was just how I was perceiving it due to the afterimage and whatnot.

I believe everyone who wants dark mode actually wants Dark Reader instead. Also, half of the "dark mode"-enabled sites are more toxic to my eyes than their bright versions. Dark Reader allows to set the desired brightness, contrast, grayscale, etc.

It's one of extensions that makes you instantly regret you didn't install it earlier.

I enjoyed trying to tell if the circle had a 3d aspect to or not, and also trying to tell if it was completely black or actually dark dark gray. I'm not sure if I'm cognitively more focused but I'm going to try it again when I need that focus.