HEheavenlyhash·Aug 28, 2016
There's a lot of great web-tech based solutions. Other commentors are covering those, so I'll suggest something different: For a take outside of the web, Golang with OpenGL bindings has a lot of promise for shipping $everywhere.
The https://github.com/go-gl github org has repos with comprehensive pre-generated OpenGL linking. When building off these, you need almost zero interaction with any host libraries. Cross compiling is a breeze. I made a demo application on Linux one afternoon to try it out: I was able to hand the code to a friend with a Mac, and he compiled and ran it first try. It's hard to understate the level of "wow" there.
Golang+GL also ships to mobile devices. There's a little squeeze dealing with the app stores, but fundamentally, build on GLES2 and you can go anywhere.
https://github.com/google/gxui/ is one library
Read the rest
on top of these foundations that went all the way to providing (beautiful!) cross-platform widgets that can be used to quickly build a desktop application.
Unfortunately, gxui, though a very attractive proof of concept, is now unmaintained. (At the same time, it's worth mentioning that it still works; there's no insane deprecation schedule or fast-moving/fast-breaking libraries to wreck your day here.) If anyone Out There is interested in renewing this project or starting more frameworks like this, I'd love to collaborate.
Overall, there's work to do here, but if you have some appetite for cutting edge, the foundation is in place to nail portability properly.
Finally, this is unrelated, but I feel compelled to give an anti-pitch to pretty much everything in C/C++ ecosystem of GUI tools: it's a landscape of last decade's promises, and today's despair. I've had comrades complain bitterly over time they've spent in codebases where the wxwidget string objects got out of hand. I have a half dozen applications suffering from various C++ linker hells on my various computers and can't recommend QT with a straight face. I still haven't gotten all my gtk3 and gtk2 configs to line up on my last debian reinstall and all the font sizes are fragged on the computer for no known reason (I literally the gtk configs in my dotfiles in git; I can't imagine how different machines drift)... And I haven't even said a word about build and shipping infrastructure required to really tackle cross-platform. I'll say it again: the entire C/C++ ecosystem of GUI tools is a landscape of despair.