I still have WebObjects 4.5.2, but could not get hands on the YellowBox

Microsoft's timing was the worst. I think they announced this a year after Swift was already announced, but before Swift was open sourced. So Microsoft wouldn't be able to deal with things like Obj-C/Swift interop which iOS developers were already jumping onto. And Microsoft's Windows 8 mobile initiative was pretty clearly a flop by this point.

Frankly, this Obj-C effort needed to be done way earlier, starting with AppKit, like back when Microsoft was panicking that OS X 10.4 Tiger was going to kick Longhorn's butt. If these tools had already been proven useful before the dawn of the iPhone, Microsoft might have had a chance of riding the iOS wave.

I think a separate objective-c runtime was ported to windows by apple for itunes and later safari to work.

If I recall correctly, iTunes was a Carbon application. They ported parts of the Carbon APIs to get it running on Windows.

QuickTime for Windows existed long before then, and Apple ported a bunch of the old Classic Mac Toolbox to Windows as part of that.

IIRC it was actually this Windows port of Toolbox that in some ways laid the foundation for Carbon - i.e. a port of the Toolbox API to what became Mac OS X.

Yes, this is correct.

Interesting stuff, I wish there was a way it would get open sourced one day.

I do remember it had its own font rendering which looked out of place on Windows.

There seem to be bunch of DLLs in iTunes and Safari with familiar names.

Would have been good if someone made bindings for them, it would have been interesting to see what MacOS like stuff could work there, even if it just python bindings.

There is also GNUStep.