Libraries like that might make it easier to get started, but they tend to limit what you can do. For example, I recently created a Desktop GUI app that could take inputs from a networking socket. How do you do that cleanly? There is often a way around limitations, for example by creating a separate thread, or by polling every so many milliseconds (which is ugly and a waste of resources). In my case, interfacing directly with Win32 without a 3rd party layer in between, it was easy to create an Event Object for the socket and am calling MsgWaitForMultipleObjects() in my message loop. Not sure what's a good solution to do this with SDL, but why should I even bother...

SDL can handle events and message passing just fine, but you would have to write most of that yourself either way, since it isn't a framework.

How do you mean it "can handle events"? I barely know SDL, but it does not look like it is super straightforward to integrate a socket:

    https://github.com/libsdl-org/SDL/blob/main/src/video/windows/SDL_windowsevents.c#L1430
    https://github.com/libsdl-org/SDL-1.2/blob/main/src/events/SDL_events.c#L403
    https://github.com/SDL-mirror/SDL/blob/master/src/events/SDL_events.c#L769

Also, it seems to create a separate listener thread just to pump the messages, which I'm not sure I like.

I could imagine SDL supports new event types by requiring the user to create _yet_ another thread, which waits for e.g. network events, then submits them to the SDL main event queue.

IMHO SDL mostly makes sense on Linux because it hides a lot of really ugly window system and GLX setup code.

On Windows (with Win32+DXGI+D3D11) and macOS (with Cocoa+Metal+MetalKit), things like setting up a window, 3D device and swap chain is just a few lines of relatively straightforward code, so SDL is by far not as useful there as on Linux.

SDL does a lot more than open windows. It gives _cross-platform_ graphics, audio, and input handling.

...which is also quite trivial with OS-native code on Win32 and macOS. Besides, the whole point of the OP's project is to not use separate dependencies.

"OS-native" implies not cross-platform.

??