If I remember Xamarin can be used to target all existing platforms, even using native looks and feel. In practice I don’t think I’ve seen it in the wild. Maybe it’s being used by Microsoft for crossplatform applications? (E.g: word, excel, OneNote, etc)

What makes React inherently un-mobile except the fact that all browsers have a touch timeout thus making UI seemingly slower?

Don’t know if it’s unmobile per se. But it’s heavy, JS wise, tends to promote SPAs, and can be quite tough to get to play well with a hybrid. The react team seems to put their effort in React Native. And that might the only framework I’d never ever ever “touch” again - pun intended:) We do use react for some mobile stuff, the issue seams to be that you still need a lot of other stuff and your app/UX still feels a bit like a 2nd class citizen

Have you ever written anything in Qt?

I've recently been using it and I am very disappointed. Some tasks that should be totally trivial are incredibly hard to get right and require platform specific solutions. Nothing unsolvable, but I was expecting a much more straightforward experience from a product that has been promoted as the cross platform toolkit since the 90s.

I co-developed a Qt C++ app of ~40,000 lines. I think the amount of platform-specific code was at most a couple of pages (handling macOS' behavior to keep an app alive when all its windows are closed).

It is not perfect, but it feels a lot more native on every platform than virtually any web or electron app I have seen (including predictable keyboard shortcuts).

I don't have a large amount of experience but when I worked with Qt I was blown away with how friendly and cross-platform it all was. It's pretty close to write once, compile to anything.

I have not. I’ve looked at it quite a bit, and it looks interesting.