Wlroots Is A New, Modular Wayland Compositor Library
The Sway compositor is currently built on its own "wlc" library that is an abstraction layer over DRM/libinput/GLESv2 and friends, but the new wlroots library is their planned replacement to offer a lower-level entry point.
Wlroots aims to make it easier to write Wayland compositors with greater code-use and interoperability between compositors by providing a base for building new compositors while also ensuring implemented compositors are interoperable via some new protocols around lock screens / on-screen keyboards / notification daemons and more.
Besides Sway working on wlroots, the Way-Cooler and Waymonad compositors have also been working on this new low-level Wayland compositor library.
Those wishing to learn more about the Wlroots project can check out its GitHub repository and the just-published wlroots whitepaper written by Drew DeVault.