This is lx, a quasi-asynchronous X library built on libaio.

The ideal here is that the application should never have to block
waiting for the server unless it actually wants to (eg, it needs a
response in order to generate its next request).  Calls that in Xlib
are blocking until a response comes back (eg, XGetInputFocus) can
instead be done in a "issue the request, call my callback when the
response arrives" style.  (Indeed, due to limitations in libaio, that
is pretty much the only workable paradigm.  There is a way to block,
but it is not very useful.)

At this writing (2023-05-20), it is still very nascent (for example,
only nine X requests are implemented so far).  What documentation there
is takes the form of comments in lx.h.