> That's not a GUI. It's something a GUI might be implemented in.
A GUI library is indeed not a GUI, but a library with which one might implement a GUI.
Usually, a GUI library does have things like actual controls implemented, but then most GUI libraries tend to present themselves as something closer to complete than most minimal.
It is unusual but not unreasonable to describe the combination of cross-platform canvas + keyboard/mouse events + audio playback as the “most-minimal GUI library”.
> A GUI library is indeed not a GUI, but a library with which one might implement a GUI.
This definition basically blurs the distinction between a "GUI library" and "graphics library". A GUI library is GTK, QT and others. A graphics library is usually divided into 3D like *GL variants and 2D like SDL or Skia or thousands of others (including this one).
> A graphics library is usually divided into 3D like *GL variants and 2D like SDL
OpenGL (what I assume you are referring to by *GL) is both 2D and 3D; it also, as one might expect of purely a graphics library, doesn't provide control events or audio and considers that something else's problem.
SDL ("Simple DirectMedia Layer") does do things like control events and audio, but does not describe itself as a graphics library, but as "a cross-platform library designed to make it easy to write multi-media software, such as games and emulators."
