Ask HN: What's the best library for making cross-platform UIs?

And even in an absolute sense, you can't confidently say that dual-licensing limits popularity in the long term. If JUCE and Qt were free for everyone, perhaps they wouldn't be able to have the full-time developers they both currently have. So maybe they wouldn't be as good, thereby decreasing popularity against competing dual-licensed libraries. This is all wild speculation, of course, but it's an example to show how their model works.

It is a shame that OP does not want to go HTML/CSS engine, because over the years, I have found that to be the easiest to update, the most reliable across platforms, and (most importantly) the easiest to support for multiple users.

I have tried (and used in production) most of the UI tookits mentioned here.

WxWindows (later WxWidgets), Tk, and FLTK are getting long in the tooth and show it.

Java (SWT, etc), looks wonky across platforms and really do not stand up to the "write once, run anywhere" promise. Also, if you have to do anything even a little out of the lines, count on keeping multiple codebases for different platforms.

Although I had a brief love affair with other VM-type platforms (Adobe Air, Silverlight), mobile mostly pushed support for them out (Air just pivoted to mobile altogether, I believe). They offered single codebase, easy updates, and easy-ish support at a cost of have the user install additional software, but worth the tradeoff in my opinion.

All of the python UI libs are a pain in the ass to distribute, update, and support.

Ultimate++ and Borland Delphi are pretty polished, but you either have to buy into their ecosystems or accept their limitations.

At the end of the day, even though Javascript is a bullshit language for desktop development, HTML/CSS engines (phonegap, Electron, sciter, etc) are just pleasant for maintaining codebase, pushing out updates, doing anything you want with the UI, and support has been a breeze.

If a language run-time or virtual machine promises, "you can write code once and run it anywhere," then they're not telling you the whole story. There's no such thing as, "cross platform." It's just a vague term that is commonly interpreted as, "major OS platforms," or, "most popular mobile platforms," etc.

Requirements are both technical and non-technical. If we just focus on the technical requirements you have to look at the data you're working with, your user, and how the program interacts in this system between the human and the rest of the world. If you're writing safety-critical mobile applications for remote workers that spend a week or more in the field you're probably not going to waste your resources on a memory-hungry VM just so that life is more convenient for you. That's starkly different to prototyping an application in a lab where convenience is paramount compared to cheap resources. I have yet to see any kind of language or run-time that lets you have it both ways.

One may say, "Yes agentultra, but what about generating the code for the highly-constrained environment from the higher-level language?" I love this approach. It's possible to generate efficient code this way. This approach still requires you to understand your target platform well enough to spot bugs in the generated code. So while it may be an approach to gain some leverage as a single developer or small team; you're still quite constrained but this DSL... the same code written for the DSL is not likely going to run well or even be considered for use by your other target platforms.

I haven't yet found a way to "escape" the reality that different combinations of hardware and OSs require different solutions for different applications. It may seem like "all you need is Java" or X, Y, or Z -- but the reality is more nuanced than that.