Cygwin/X source code is contained in, and distributed with, the X Window System source code tree.
The source code for the packages distributed via Cygwin setup is also available via Cygwin setup. This may have multiple patches applied on top of the X Window System source code, and should be the starting point for new developers.
Read-only git access to the X Window System source tree is also available.
The CYGWIN branch exists in git for historical reasons. Current development follows the mainline.
$ git clone git://git.freedesktop.org/git/xorg/xserver |