As far as I know, there are not "React" patents. Facebook has many patents, but I haven't seen anyone point to any patent which specifically involves React. The author of https://medium.com/@dwalsh.sdlr/react-facebook-and-the-revok... did a cursory search and concluded:

"To determine patent liability asserted by Facebook, we need to assess Facebook’s various patents and their claims. Or more specifically, which Facebook patents claim any underlying React technology? As the above illustrates, this is not a trivial question. But in my cursory search, that answer is zero. (Again, if you have examples, send them over.) Moreover, it would suprise me given the USPTO and courts general disfavor of software patents if Facebook had anything patentable in React, much less actually patented. The prior art of componetization, diffing, and virtual doms circa 2013 is very well known, and I don’t see any applicable continuation patent applications."

What patents regarding React exist and which of these are held by Facebook?

That’s part of the strangeness of this whole debate: nobody knows. The list of Facebook’s patents is public, but I’m not aware of anyone publicly going and doing the research to see which of them, if any, are likely to apply to React. (Possibly because of the whole treble damages thing - along with the fact that Facebook has thousands of patents to look through.) Facebook’s lawyers may or may not have done that research, but it’s in their best interest to keep things as vague as possible (at least up to the point where they actually threaten to sue someone), to avoid workarounds.

Facebook can sue anyone for patent infringement whether or not they use React. Many people seem to believe that they can't ever be sued by Facebook for any reason as long as they don't use React although that is not the case.

The list of contributors to React, and Facebook patent holders are both public. If someone cared to they could cross reference both and narrow down the search area.

The ones that haven't been finalized by the patent office yet.

Do you really think they don't have a pending patent for some part of React? I bet they've got at least one covering virtual DOM.

And don't worry about prior art. As long as some part of it can weasel through a patent it's enough. Doesn't have to be the whole thing.

It's unlikely Preact infringes on a theoretical React patent. It's not a reverse-engineered reimplementation of React, but rather a library that happens to have a similar public API. APIs are not patentable, thus Preact's implementation is no more likely to infringe than Angular or Vue's.

If you can't fork React then why is it open source? Their license just says you can't sue Facebook for patent infringement. That's it.

Sounds to me more like "shared source" that MS used to push.