What’s wrong with something like Qt or wxWidgets for native crossplattform GUI frameworks?
10-15 years ago it was possible to reliably clock a Qt on Mac app before it finished launching, simply from the app using the wrong hourglass cursor. (The old cursor that had once been a hallmark of classic Mac OS apps ported through Carbon frameworks, and which disappeared from daily usage when the Finder was finally rewritten atop the NeXTSTEP-derived Cocoa frameworks.)