/usr/share/man/man3/XtIsRectObj.3.gz is in libxt-dev 1:1.1.4-1+b1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man3/XtClass.3.gz
/usr/share/man/man3/XtClass.3.gz