Support push/pop window title sequence (CSI 22/23)

[mitchellh]

From #2667

Source: https://invisible-island.net/xterm/ctlseqs/ctlseqs.html

This is a fairly well defined sequence that is supported -- at least in its most basic push/pop form -- by many terminals (Kitty, iTerm, xterm). It can be combined with the smcup and rmcup terminfo entries to push pop by default (xterm does this) on alt screen.

The primary sequences we should support:

         Ps = 2 2 ; 0  ⇒  Save xterm icon and window title on stack.
         Ps = 2 2 ; 2  ⇒  Save xterm window title on stack.
         Ps = 2 3 ; 0  ⇒  Restore xterm icon and window title from
       stack.
         Ps = 2 3 ; 2  ⇒  Restore xterm window title from stack.

We can ignore the "icon title" variants as they aren't used by any modern window managers that I know of. We can implement those later as needed.

For bonus points, we should consider this:

      XTWINOPS 2 2  (save/push title) and 2 3  (restore/pop title)
      accept an optional third parameter for direct access to the
      stack.  Parameters in the range 1 through 10, may be used to
      store the title into the stack or retrieve the title from the
      stack without pushing/popping.

cc @lilyball

[mitchellh]

Some details from reading the xterm source:

• The stack size is 10 and is non-configurable
• On push and pop, the index is 1-indexed if specified
• On push, if an index is specified, it is used even if the stack isn't that large (i.e. index 7 but no pushes)
• On push, an index <= 0 means 0 (push op)
• On push and pop, the index is always % 10 so values greater than 10 are allowed and just mod
• The stack is screen-specific (primary/alt have different title stacks)
• Full reset (ESC c) does not reset the title stack

[mitchellh]

I added VT parsing for this in #2669. Implementation still needs to be done.

[lilyball]

xterm uses index 0 in its smcup/rmcup entries. Does that mean the same thing as omitting the index, since you said it’s a 1-based index?

Which way does the stack grow? Is index 1 always the last-pushed item, or does each push take the next slot (and then shift everything once it fills up)?

[lilyball]

Oh also to note, the title stack should include the proxy icon data as well.

[mitchellh]

xterm uses index 0 in its smcup/rmcup entries. Does that mean the same thing as omitting the index, since you said it’s a 1-based index?

Yes.

Which way does the stack grow? Is index 1 always the last-pushed item, or does each push take the next slot (and then shift everything once it fills up)?

It's actually not implemented as a stack at all in xterm, it's implemented as a circular buffer. I haven't verified this with shell scripts, just reading the source code. I'm a doofus. It is written as a circular buffer but the "top" index is moved so it is a stack by shifting all the elements (removing the oldest).

Oh also to note, the title stack should include the proxy icon data as well.

I'll have to think about this. Obviously xterm doesn't know about pwd reporting at all so it doesn't store this, but I'll have to look into other terminals to see what they do about this. Are we also 100% sure that Terminal.app also does this?

[mitchellh]

Updated my discoveries from reading the source to add that the title stack is per-screen (primary/alt have their own stack).

[lilyball]

Are we also 100% sure that Terminal.app also does this?

I am 100% sure that Terminal.app is restoring both window title and proxy icon state on a mode 1049 exit back to the values they had before entering mode 1049. I do not believe it's implementing any sort of stack, since it does not use the 22/23 sequence at all (and the macOS terminfo database doesn't include the 22/23 sequence in smcup/rmcup). Here's a script I just ran to test this:

printf '\e]7;file:///Users/lily/Documents\a\e]2;foo\a'
read
printf '\e[?1049h'
read
printf '\e]7;file:///Users/lily/Music\a\e]2;bar\a'
read
printf '\e[?1049l'
read

Since you're implementing the title stack instead of tying this to mode 1049, then the title stack should include the proxy icon. I just looked up what the "icon title" thing is in case it made more sense to use that, but nope, that was a thing for the title to use when minimizing a window into an icon.

Incidentally, Terminal.app also supports OSC 6 for the "document" path, which is like OSC 7 except it treats the path as a file instead of a directory. It stores them separately but displays the document path if set (i.e. if you set both, it shows the document path, and if you then unset the document path, it shows the working directory). Ghostty does not currently support OSC 6 (maybe it should? I'm not sure right now if any software actually uses it; vim is actually sending an OSC 7 in Ghostty when editing a file even though it doesn't do that in Terminal.app and I haven't dug into that yet) but if it does support that then it too needs to be part of the title stack.

[mitchellh]

Since you're implementing the title stack instead of tying this to mode 1049, then the title stack should include the proxy icon. I just looked up what the "icon title" thing is in case it made more sense to use that, but nope, that was a thing for the title to use when minimizing a window into an icon.

I don't think this is the way I should implement this. Since this is a macOS specific thing, if we decide to mimic it (since we support the proxy icon we maybe should), we should probably relegate this to aport-specific behavior rather than being part of libghostty at all.

The fact is that no other terminal persists the pwd as part of the title stack or mode 1049. This is purely a Terminal.app oddity. For example, it'd be really strange if this sequence occurred: (1) set pwd A (2) push title (3) set pwd B (4) pop title (5) report pwd and get "A" back. Every other terminal in the world (importantly xterm) would return "B" here.

Therefore, I think the cleanest thing to do would be to separate it from this issue altogether and consider it a wholly separate feature. And I think the right way to design that one would be to implement apprt actions when alt screen is entered/exited, and have the macOS AppKit app handle pushing/popping this.

The end result would be that the macOS app proxy icon behaves like Terminal.app, and the core VT emulation behaves like the rest of the world.

For the sake of this issue, I'll say this issue should focus solely on CSI t 22/23 with xterm compatibility.

[mitchellh]

Another update from reading the xterm code, verified with a shell script (below): full reset (ESC c) does NOT reset the title stack. That is surprising.

#!/usr/bin/env bash

function csi() {
  echo -ne "\033[$1"
}


echo -ne "\033]2;Outside\a"
csi "22;0t"
csi "?1049h"
echo -ne "\033]2;Inside\a"
echo -ne "\033c"
csi "?1049l"
csi "23;0t"
read

[lilyball]

The fact is that no other terminal persists the pwd as part of the title stack or mode 1049. This is purely a Terminal.app oddity.

What other terminal even supports OSC 7? I'm guessing Kitty does simply because the Ghostty bash script uses kitty-shell-cwd:// instead of file://, but iTerm doesn't. OSC 7 was invented by Terminal.app, so it's the authority on how it's supposed to behave. If you want to tie the proxy icon restoration to the mode 1049 switch instead of the title stack I guess that's fine since either way the behavior is tput rmcup restores it, it just seems a bit odd to separate out, and means you can't do something like use the title stack codes separately from mode 1049 to bracket subshells (which otherwise would be a fairly neat way of ensuring the pwd is restored when exiting the subshell).

report pwd

I'm not aware of any code to report the pwd? It's hardly needed since processes already know their own pwd.

[mitchellh]

What other terminal even supports OSC 7?

Almost all of them nowadays (libvte supports it so that makes pretty much every Linux terminal, Kitty, WezTerm, Foot, etc.). More support it than don't in terms of terminal that are used by people. 😄

OSC 7 was invented by Terminal.app, so it's the authority on how it's supposed to behave.

There's no real authority on how terminals work unfortunately. terminal-wg is a kind of authority and they've taken over the OSC7 specification. You can take up any arguments with them. Ghostty's stance on VT sequences has always been xterm followed by the majority of other terminals.

I'll test to see if any other terminals associate pwd with 1049 or push/pop, but I still stand by that until more research is done, we should keep it separate from this issue. To be clear, that's not a no, I'm just not willing to make a hasty VT behavioral decision without more research into the issue across different terminal emulators, roping in those maintainers if necessary.