alt Hacker NewsJust to clarify: despite its name, React has no reactivity engine at all and re-renders unconditionally on any state changes or parent re-renders.
You have to opt-in to prop-diffing conditional re-renders (which I wouldn't call a "reactivity engine" either) per component, via React.memo.
And then you also have to opt-in to prop memoization for non-primitives for the prop-diffing not to be useless.
These re-renders might not result in any actual DOM changes since there is an additional vDOM diffing step from the resulting render (which, again, I wouldn't call a "reactivity engine").
I am probably just misguided but multiple things you say you "wouldn't call a reactivity engine" only exist to support updating DOM (reactivity) efficiently. So what would you call them? They only exist to update the DOM, which is itself the reactivity we all want. Technically you could just getElementById and update it, but react has what I call a whole "reactivity engine" to do it for you.