bug#54027: Wishlist: Support full CSI u specification for terminal input

[Richard Stallman]

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  > The specification is implemented by iTerm2, which is a popular
  > GPL-licensed terminal for macOS:
  > https://iterm2.com/documentation-csiu.html

We shouldn't spend time adding a feature to GNU Emacs
for the sake of running it on MacOS.

  > The GPL-licensed mintty terminal for Windows also has support:
  > https://github.com/mintty/mintty/wiki/Keycodes

Nor Windows.  We should not implement features that are of no use
on the GNU system.

  > A variant of the specification is implemented by kitty, another
  > GPL-licensed terminal on GNU/Linux and macOS:
  > 
https://sw.kovidgoyal.net/kitty/keyboard-protocol/#comprehensive-keyboard-handling-in-terminals

Since kitty runs on GNU/Linux, maybe it is worth implementing
this variant of the specification, for the sake of kitty.

It's a matter of weighing the work involved (development and
maintenance) against the benefits.

We could wait and see if this protocol catches on, on GNU/Linux.
If it does, it will surely be worthj supporting.

-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)