An Xlib compatibility layer implemented on top of the Haiku API, in order to run X11 applications on Haiku without an X server.
Xlib‘s API is relatively low-level, but it is just high-level enough that it can be emulated on top of a higher-level API like Haiku’s.
At present, it provides “most” commonly-used Xlib APIs, but many of them are stubbed or incomplete implementations. (GTK, with some hacks, can compile, link, and open a window before it runs in to missing functionality.)
This is crazy person work by Haiku developer waddlesplash. He also posted continuously updated progress thread on the Haiku website, which provides a lot more detail about the process, the current state, and possible future plans.