The X Composite Window Manager Library and XtoQ come out of research projects at Portland State University and is now being continued through an Endless Vacation of Code project.
As explained by Jess VanDerwalker, "The goal of XCWM is to create a C library that acts as a layer between XCB and OS specific window manager code that facilitates running X client windows as OS native windows on platforms such as OS X, Microsoft Windows, or Wayland. Through XCWM would allow developers to share more code, improve maintainability, and eventually remove some of the specific DDX's within the server code base. XtoQ, which has been developed concurrently, is a proof of concept window manager application developed for OS X which uses XCWM."
The code for the XCWM is developed on FreeDesktop.org. For those wishing to find out more about XtoQ and XCWM, the XDC2012 video is embedded below.