Quick writeup of TELOPT_MPLEX, the nonstandard telnet option for doing popups on the terminal, 
used by Cryosphere and Crystal since 2002 or whenever. By morwen@evilmagic.org.

See https://musicmud.org/crystal/overlay-demo.png to see it in action

#define TELOPT_MPLEX  0x70

server sends
  IAC WILL TELOPT_MPLEX

client sends
  IAC DO TELOPT_MPLEX

server can then use these commands.

#define MPLEX_SELECT  0x71
#define MPLEX_HIDE    0x72
#define MPLEX_SHOW    0x73
#define MPLEX_SETSIZE 0x74

(yes i know these don't need to be adjacent to the option i don't know why i did it like that)

valid screen IDs are between 0 and 254. 0 means the "main" screen. Crystal actually only
supports 0 and 1 at the moment.

  IAC SB TELOPT_MPLEX MPLEX_SELECT [screen] IAC SE

This selects a new terminal context. These should have completely different ANSI parsing state
machines, which should restore when you go back to an old one, etc etc.

  IAC SB TELOPT_MPLEX MPLEX_HIDE [screen] IAC SE

This should hide the given screen. Only valid for non-main.

  IAC SB TELOPT_MPLEX MPLEX_SHOW [screen] IAC SE

This should show the given screen. Only valid for non-main.

  IAC SB TELOPT_MPLEX MPLEX_SETSIZE [screen] WIDTH[1] WIDTH[0] HEIGHT[1] HEIGHT[0] IAC SE

This sets the size of the given screen. Same format as the NAWS subnegotation.  Only valid for non-main.