XQuartz 2.7.9_rc1 is available for download. This update contains a workaround for an issue that users have been reporting on El Capitan. If you're on El Capitan and have been seeing OpenGL surfaces remain onscreen after they should have been dismissed, please give this update a try and report back to the developers.

http://www.xquartz.org/releases/XQuartz-2.7.9_rc1.html