You had been flagged for whatever reason…not sure why.
> several Apple software like Safari 5 for Windows and iTunes already ship with macOS UI libraries ported to Windows.
Actually, the situation is kind of the opposite: Safari and iTunes eschew the use of macOS's UI libraries. Safari, in particular, is actually just WebKit (which has no "UI") wrapped up in a Windows-style chrome to appear native. iTunes itself, even on macOS, is some sort of weird C++ hybrid abstracted API that's not Cocoa (which is why it looks so "off" on macOS). This, along with a version of Foundation/CoreFoundation, allows it to work on both platforms–though not very well, I'll admit.
> several Apple software like Safari 5 for Windows and iTunes already ship with macOS UI libraries ported to Windows.
Actually, the situation is kind of the opposite: Safari and iTunes eschew the use of macOS's UI libraries. Safari, in particular, is actually just WebKit (which has no "UI") wrapped up in a Windows-style chrome to appear native. iTunes itself, even on macOS, is some sort of weird C++ hybrid abstracted API that's not Cocoa (which is why it looks so "off" on macOS). This, along with a version of Foundation/CoreFoundation, allows it to work on both platforms–though not very well, I'll admit.