What is the equivalent to Win32 on Linux? I know wine and Proton, but I looking for a native solution. This would make vendors write more gui software for Linux if there is a standard api.

Just use Electron.

Win32 was an early widget set for Windows, Microsoft created a new one every few years until 2012, when it came out with the unwanted Metro. Since then it's been unclear what widget set Microsoft wants users to use, "use Electron" seems to be as official any answer.

In the case of Linux there are an even larger number of widget sets: see

https://en.wikipedia.org/wiki/List_of_widget_toolkits

Instead of systems programming interfaces in Win32, you use POSIX. You know, files, shared memory, semaphores, threads, processes, pipes, ...

On the graphics side, though: classically, it's been XWindow: a base library called XLib talks the X protocol and does lower level manipulation; widgets implemented on top of that. With XLib you do things like repaint a window in response to expose events, somewhat analogously to a WndProc getting events.

Wayland has its own libraries.

Then you have things like working with framebuffers and SDL and whatnot.

Motif / Lesstif / CDE, I guess?

If I understand what you're looking for, GTK. If it's a linux desktop, there's a super high chance that GTK is both installed and familiar to the user.

However...

> This would make vendors write more gui software for Linux

What makes you think vendors want the native look and feel?

Even on Windows, there's a long history of apps wanting to feel "unique". That's what they want: full customization to fit their branding.

There is none

Ncurses?

glibc -> wayland -> gtk

I started a war answering there. You want a monolithic standard api? Great you do it.

You know what you'll get? Even more options.