Why do you claim Nuklear is not a "true single-header style library"? And who gets to say what that even means?
Nuklear is a full-sized library that can be compiled into and used as one file. However, RGFW and STB are designed to be lightweight and minimalistic. That's the distinctive part of single-header libraries, also known as STB-style libraries.
To be clear, I don't mean this to hate on Nuklear, it's a cool UI library, but its design doesn't match the single-header format.
How is it not designed to be lightweight and minimalistic?
How do you think the design differs from your idea (which is what exactly?) of a "single-header format"?