If you're concerned about React, then Preact is actually significantly worse.
Consider: The risk for React is based on 1) speculation that Facebook has patents on core React concepts and 2) the fact that patent grant that comes with React terminates in some situations if you end up in conflict with Facebook. So for safety, it might be better to make sure you use code that Facebook has no patents on, which (if (1) above is true) means "code that is nothing like react at all".
A React clone that doesn't have the patent grant at all is vulnerable to all the same issues React itself is, plus more. It's strictly more dangerous than using React itself.
(Unless, of course, we assume there's some bit of React that's subject to a Facebook-owned patent that would not also apply to Preact. But having spent a few minutes thinking about it, nothing leaps out at me.)
> Its internals are fundamentally different and not a derivative work.
That would be a strong defence to a copyright claim. We're talking about patents, where there is no concept of "derivative works", and what matters isn't implementations but broad concepts.
> Software interfaces cannot be patented
Right, but what makes Preact concerning isn't that it offers the same API, but that it is built using the same underlying concepts.
Preact renders independent, encapsulated, nested, functional-ish components, usually but not always written in JSX, into a virtual DOM. Components are rerendered to the virtual DOM when their inputs change, and then diffing is used to efficiently sync the virtual DOM with the browser DOM.
If Facebook has some sort of patent covering the virtual DOM (although thankfully as far as I know, they don't), it's hard to see how it would not also cover Preact. And this is true even if, as I assume is the case, Preact shares 0 lines of code with React.
Edit: To be clear, I don't believe the patent situation is concerning, I think Preact is a great project, and I may well be using it in production soon. But if I was worried about Facebook's patent portfolio, I would be worried about using Preact.
Understandable, and agreed about my point being for copyright as opposed to patents. It's interesting to note that, since the majority of DOM rendering libraries now employ some variant of Virtual DOM (react, preact, mithril, glimmer/ember, angular, even polymer!), a patent claim covering the concept of Virtual DOM itself would likely be invalid - the presence of so many prior and contemporary implementations of that paradigm make would invalidate it on the grounds that it cannot be "Non-obvious" or "non-novel". My knowledge of this stuff is largely gleaned from Wikipedia and thus not to be trusted, but it seems like there would be little grounds for a patent suit over VDOM. Also interesting to note: there are few (if any?) other companies directly associated with any of the other VDOM implementations - can a patent claim even be made against an Open Source project?
Consider: The risk for React is based on 1) speculation that Facebook has patents on core React concepts and 2) the fact that patent grant that comes with React terminates in some situations if you end up in conflict with Facebook. So for safety, it might be better to make sure you use code that Facebook has no patents on, which (if (1) above is true) means "code that is nothing like react at all".
A React clone that doesn't have the patent grant at all is vulnerable to all the same issues React itself is, plus more. It's strictly more dangerous than using React itself.
(Unless, of course, we assume there's some bit of React that's subject to a Facebook-owned patent that would not also apply to Preact. But having spent a few minutes thinking about it, nothing leaps out at me.)