Mouse movement is signaled in the terminal via ANSI escape sequences, which are just in-band character sequences which your terminal is smart enough not to display, so Telnet itself doesn’t actually have anything to do with it. As far as Telnet knows, they aren’t different from any other characters.

There are some oddities in the telnet version, such as it conflating XTerm and PuTTY, and it presenting a bizarre timeout message when presented with a blank TERM environment variable. I wouldn't be surprised if it somehow thinks that NVTs cannot have more than 256 columns.

Alas, that's the part that has no source available. (The TELNET server class library has published source, buried in a hyperlink in a comment to a closed issue from 2019, but the actual program built to use that library has not.)

Thanks! Previous threads macroexpanded:

Telnet Mapscii.me - https://news.ycombinator.com/item?id=44572328 - July 2025 (1 comment)

MapSCII – A Braille and ASCII world map renderer for the console - https://news.ycombinator.com/item?id=39975887 - April 2024 (23 comments)

MapSCII – The Whole World in Your Console - https://news.ycombinator.com/item?id=27042629 - May 2021 (43 comments)

MapSCII: World map renderer for your console - https://news.ycombinator.com/item?id=20375881 - July 2019 (24 comments)

MapSCII – Braille and ASCII map renderer for the console - https://news.ycombinator.com/item?id=14254165 - May 2017 (65 comments)

Won't go offline before me going offlife ;)

Oh man, me too. SCII = StarCraft 2, and "maps" was such a typical term in StarCraft. I loved it. Had a great time with it, 20 years ago.