Is this any relation to Nano-X? http://microwindows.org/
TinyX is an XServer, it only talks to VESA compatible graphics hardware and provides an X11 interface. It's intended for PCs.
Nano-X is a graphics library that can talk to frame buffers, graphics hardware, or an XServer and can provide some level of X11 interface as well as a Win32 API. It's intended for embedded machines.
I used it for a bit 25 years ago when we were using it as the GUI for a Linux based tablet because X took too much memory...
But it only requires a driver that can handle a minimum of reading/setting individual pixels and draw horizontal and vertical lines and manage the palette (of course if whatever you're drawing on supports more than that, you'll get far better performance if you implement e.g. blits).
We were using it without any shims, and had a very tiny GUI lib running directly against the Nano-X API, and contracted Opera to port their browser to it as well to use on our tablet.
It was a fun project, but way too early - the hardware we built was beautiful but so extremely constrained, leading to put a huge effort into shrinking everything. Never made it to mass production.
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!