/usr/share/man/man3/QLength.3.gz is in libx11-doc 2:1.6.2-3+deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man3/AllPlanes.3.gz
/usr/share/man/man3/AllPlanes.3.gz