Have read those Pi3D
posts many time, still don't really understand it.
is a layer, so is Pygame and SDL2 etc, confused by so many layers, which does what?
pik33 normally has a lower level understanding down to assembly level.
The reason is because I came across the EGL_surfaceless option in mesa.
If the compositing is done using EGL on the Pi4 then non x11 surfaceless with transparency could be an option.
Mesa could be used without the need for x11 which makes headless OS's with simple GUI's much smaller.
Eventually I would like to use mesa to do GUI's on baremetal Pi4's.
And then perhaps direct without mesa although the GLSL shader compiler would be handy.
Understanding transparency on Pi4's is a requirement for the layers I want to use in an accelerated GUI.
Edit - been rereading those Pi3D
posts, it make a tiny bit more sense now, this error is something I came across in mesa EGL_surfaceless.
I think SDL2 can do surfaceless on Pi's.
As you say OpenGL or DRM/KMS.
For Pi4 the "underlying native platform window system" is either OpenGL, or DRM/KMS.
Rereading that post and now I remember where I got up to, thanks, my memory is going.
No libGLX in Gentoo64 I think was my current guess.
I'm dancing on Rainbows.
Raspberries are not Apples or Oranges