While there are some projects that implement partial versions of Xlib APIs on non-X11 platforms for compatibility (most notably, Tk), I don't know of any others sophisticated enough to run Cairo and GTK applications (albeit with a few hacks). So I figured this might be technically interesting.

As of now it just implements the base Xlib APIs, it doesn't implement XRender, GLX, XInput2, etc. (though it's more than possible, I just didn't see a need yet.) Cross-program interaction is also very limited, and there are plenty of X11 features that likely can't be implemented; but this works sufficiently well that GTK3 is now in Haiku's default package repositories, with GIMP and Inkscape atop it to boot, using this.

And for the fun of it, I did try to compile Xnest (the X.org server variant running on top of Xlib) on top of this; it crashed on startup in the keyboard handlers. However, that could potentially be because I didn't have the necessary data files and not because of anything missing in Xlibe; I didn't investigate too far (or it could be an actual incompatibility; Xlibe's keyboard subsystem is rather primitive and is most of the reason I had to patch GTK for full functionality.)

(A few more screenshots, including GTK and WINE running atop Xlibe: https://www.haiku-os.org/blog/waddlesplash/2022-01-10_haiku_...)