To install xorg-libXext, run the following command in macOS terminal (Applications->Utilities->Terminal)
sudo port install xorg-libXext
To see what files were installed by xorg-libXext, run:
port contents xorg-libXext
To later upgrade xorg-libXext, run:
sudo port selfupdate && sudo port upgrade xorg-libXext
Reporting an issue on MacPorts Trac
The MacPorts Project uses a system called Trac to file tickets to report bugs and enhancement requests.
Though anyone may search Trac for tickets, you must have a GitHub account in order to login to Trac to create tickets.