I agree this should be built-in. Many apps have their own "focus mode", but I wanted something that works uniformly across all apps. There is a full-screen mode in macOS, but it stretches the widow and context to full width, which doesn't pan out too well in some apps (lots of empty space or lines that are too long to read comfortably). So this preserves the window width but extends the height when focused, which works well for my daily drivers. The "fade" is somewhat unusual too, which means everything else is darkened close to black, which I find visually suppressing things and reduces distraction.
By the way, repo has an animated demo in case you want to see how it works.