If we look at this directory:
https://github.com/ghaerr/microwindows/tree/master/src/drive...
Most notably the source files that start with 'scr_', and of those most notably: scr_sdl2.c, scr_win32.c, scr_x11.c, scr_djvesa.c, scr_fb.c -- we see that this windowing system can apparently run on top of an existing windowing system, whether that system is SDL2, Win32, X11, VESA, Linux's framebuffer -- or several others.
Which makes it interesting and worthy of study...
Note that I am sure there are probably a whole lot of other windowing systems out there that also support these, let's call them "back-end" (for lack of better terminology) pre-existing windowing systems.
In other words, a windowing system -- on top of another windowing system...
Sort of like running X on top of Win32, or Win32 on top X...
But the posibilities of higher level and lower level windowing system are really unlimited -- mix and match, basically...
In conclusion -- excellent link!