Support for Kitty multiple-cursors protocol

[Delapouite]

Hello

This thread is just to share a new initiative from Kitty's author, who has come up with a new protocol to express multiple cursors at the terminal via escape codes:

kovidgoyal/kitty#8927

(mawww is already tagged in it.)

I know my precedent thread was already Kitty related, and I apologize. I'm not trying to oversell this terminal to anyone, it's the just the one I'm currently using and I like that it tries to push the boundaries of what's possible for TUI apps.

[krobelus]

I'm test-driving this (with foot's WIP implementation), see the kitty-multiple-cursors branch in https://github.com/krobelus/kakoune

here's some screenshots
(vertical bar before and after selection, and the unfocused state).
The first one feels a bit odd, we'll see.

[krobelus]

One of the problems I found with my current implementation is that cursors will be painted over UI widgets such as completion menus, which looks wrong.

We could work around this but I wonder if it would be better if we implemented this via normal faces, like we can already do for underline.
I need to find out what the upsides of native cursors are, I guess:

1. automatic shape change when the window loses focus (but we can do that easily in Kakoune, using focus notifications (CSI O))
2. "cursor trail animations" mentioned in feat(terminal): support native terminal cursor rendering #5348
3. the reasons mentioned in https://github.com/kovidgoyal/kitty/blob/master/docs/multiple-cursors-protocol.rst, e.g. "[Unicode glyphs are] sub-optimal since actual cursors implemented by the terminal have many niceties like smooth animation, auto adjust colors, etc."

Consider this config snippet that mimics Neovim behavior:

• insert mode gets a vertical line on the left edge,
• replace mode underlines the whole selection(s)

hook -group my global ModeChange push:.*:insert %{
	set-option -add window ui_options terminal_cursor_native=steady-bar
	face window PrimaryCursor default,default
	face window SecondaryCursor default,default
	face window PrimaryCursorEol default,default
	face window SecondaryCursorEol default,default
}
hook -group my global ModeChange pop:insert:.* %{
	set-option -remove window ui_options terminal_cursor_native=
	unset-face window PrimaryCursor
	unset-face window SecondaryCursor
	unset-face window PrimaryCursorEol
	unset-face window SecondaryCursorEol
}
hook -group my global ModeChange push:.*:next-key\[replace-char\] %{
	face window PrimarySelection default,default+u
	face window SecondarySelection default,default+u
	face window PrimaryCursor red,default+u
	face window SecondaryCursor blue,default+u
	face window PrimaryCursorEol default,default+u
	face window SecondaryCursorEol default,default+u
}
hook -group my global ModeChange pop:next-key\[replace-char\]:.* %{
	unset-face window PrimarySelection
	unset-face window SecondarySelection
	unset-face window PrimaryCursor
	unset-face window SecondaryCursor
	unset-face window PrimaryCursorEol
	unset-face window SecondaryCursorEol
}

Wouldn't it be nice if we could use something like +u (underline) also to get a vertical line?
We don't want to do that with the existing RFC, because that would hide the
fact that vertical-line is actually intended only for cursors.. and also it
wouldn't solve the problem that cursors are painted over completion menus.