/usr/share/man/man3/XmbufCreateStereoWindow.3.gz is in libxext-dev 2:1.3.0-3ubuntu0.2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man3/Xmbuf.3.gz
/usr/share/man/man3/Xmbuf.3.gz