Blackberry BB10 was based on QNX. The UI "feel" was fantastic.

Yes, it was fantastic - minimal, elegant, simple, fast, had low learning curve and everything had a description - even icons in action bar[0]. As their competitors, namely iOS7+ and Android 5+ totally replaced gradients into flat design I was very welcoming the balanced UI redesign on my Z10[1]. However, autocompletion on their touch keyboard was probably the best [2], you had 3-5 suggestion that were spread around on the keyboard and to use them I had to place a finger below and flick it up. On BB Passport it was even more pleasant with physical keyboard - it had small gesture sensor and reacted on swipes in the air above the keyboard.

On the developer side, documentation was also amazing. There are tens of full, working example applications. BB10 used QT behind the scenes (Cascades UI). It had also a Python (2.x) binary on the device among other typical UNIX programs.

[0]: https://developer.blackberry.com/devzone/files/design/bb10/i...

[1]: https://developer.blackberry.com/devzone/design/bb10/10_3_vi...

[2]: https://developer.blackberry.com/devzone/design/bb10/keyboar...

I've been involved with Qt for a long time and got a Z10 as a conference freebie, but the great developer documentation is actually news to me! I prefer to just use phones, I develop for PC hardware for fun and embedded platforms for money these days. At the time, some of my coworkers helped with porting Qt to QNX. I worked on a different Qt project on QNX and I was also duly impressed by the technical quality and elegance of the OS.

The BB10 UI was built on something like QML (the language and runtime) from Qt 4 with their own UI elements and an OpenGL based backend from the acquired company The Astonishing Tribe. They had animations e.g. for slider switches running in the render thread, perfect 60 fps. Qt Quick (the UI framework based on QML, colloquially called "QML") only got an OpenGL backend in Qt 5.

Another very good Qt-based phone OS (after the Nokia N9, got one of these at a conference as well) that failed :(

By the way, the Ford Sync 3 IVI ("in-vehicle infotainment system") is also based on QNX and Qt and it received fairly good reviews. I think I made some tiny contribution to it, if only helping a coworker with something.

Are there any open source real-time operating systems?

None with a GUI. I never released mine, and it's so far behind the times now it would take major work to get it ported to the 64 bit era and even then it would still be a C based OS.

This would be one way in which the Rust crowd could really make a difference.

Redox seems like a big effort that has at least some of the properties you're looking for:

https://www.redox-os.org/

But Redox is not a real time OS is it? I've not seen it written as part of the design or as a future goal.

It's a microkernel with even the drivers running in userspace, and has its own graphics stack. Seems the ideal setting to go for realtime requirements for the UI. I don't know if they've done anything in that direction but it seems the most well positioned to fulfill the requirements jacquesm was hoping for.

If they can see it through then that's a pretty good development. But there is more to QnX than just 'realtime' and 'microkernel', their whole philosophy about how a cluster of machines should work, the services location and so on is very well thought out. I'd be happier if Redox took some more lessons from Erlang and QnX on board than to do a re-invention. We have a lot of knowledge about systems like that lying around and re-inventing the wheel is fun but also ultimately will lead to having to re-learn those lessons the hard way.

Perhaps you should lend a hand then? :)

Not for large systems, but in the embedded space there is Nuttx (now managed by Apache foundation).