Test balloon for ST OSC terminator

Use \e\\ as sequence terminator for the first OSC 133 prompt start
marker.  The intention is to find out if a terminal fails to parse
this. If needed, the user can turn it off by adding "no-mark-prompt"
to their feature flags; but the shell is still usable without this.

Part of #12032

doc_src/terminal-compatibility.rst

@@ -8,6 +8,7 @@ while others enable optional features and may be ignored by the terminal.
 The terminal must be able to parse Control Sequence Introducer (CSI) commands, Operating System Commands (OSC) and :ref:`optionally <term-compat-dcs-gnu-screen>` Device Control Strings (DCS).
 These are defined by ECMA-48.
 If a valid CSI, OSC or DCS sequence does not represent a command implemented by the terminal, the terminal must ignore it.
+For historical reasons, OSC sequences may be terminated with ``\x07`` instead of ``\e\\``.
 
 Control sequences are denoted in a fish-like syntax.
 Special characters other than ``\`` are not escaped.
@@ -216,13 +217,13 @@ Optional Commands
      - Disable bracketed paste.
    * - .. _term-compat-osc-0:
 
-       ``\e]0; Pt \x07``
+       ``\e]0; Pt \e\\``
      - ts
      - Set terminal window title (OSC 0). Used in :doc:`fish_title <cmds/fish_title>`.
-   * - ``\e]2; Pt \x07``
+   * - ``\e]2; Pt \e\\``
      - ts
      - Set terminal tab title (OSC 1). Used in :doc:`fish_tab_title <cmds/fish_tab_title>`.
-   * - ``\e]7;file:// Pt / Pt \x07``
+   * - ``\e]7;file:// Pt / Pt \e\\``
      -
      - Report working directory (OSC 7).
        Since the terminal may be running on a different system than a (remote) shell,
@@ -235,20 +236,20 @@ Optional Commands
        This is used in fish's man pages.
    * - .. _term-compat-osc-52:
 
-       ``\e]52;c; Pt \x07``
+       ``\e]52;c; Pt \e\\``
      -
      - Copy to clipboard (OSC 52). Used by :doc:`fish_clipboard_copy <cmds/fish_clipboard_copy>`.
    * - .. _term-compat-osc-133:
 
-       ``\e]133;A; click_events=1\x07``
+       ``\e]133;A; click_events=1\e\\``
      -
      - Mark prompt start (OSC 133), with kitty's ``click_events`` extension.
        The ``click_events`` extension enables mouse clicks to move the cursor or select pager items,
        assuming that :ref:`cursor position reporting <term-compat-cursor-position-report>` is available.
-   * - ``\e]133;C; cmdline_url= Pt \x07``
+   * - ``\e]133;C; cmdline_url= Pt \e\\``
      -
      - Mark command start (OSC 133), with kitty's ``cmdline_url`` extension whose parameter is the URL-encoded command line.
-   * - ``\e]133;D; Ps \x07``
+   * - ``\e]133;D; Ps \e\\``
      -
      - Mark command end (OSC 133);  Ps is the exit status.
    * - .. _term-compat-xtgettcap:

src/terminal.rs

@@ -9,6 +9,7 @@
 use crate::threads::MainThread;
 use crate::wchar::prelude::*;
 use bitflags::bitflags;
+use once_cell::sync::OnceCell;
 use std::cell::{RefCell, RefMut};
 use std::env;
 use std::ffi::{CStr, CString};
@@ -384,7 +385,12 @@ fn osc_133_prompt_start(out: &mut impl Output) -> bool {
     if !future_feature_flags::test(FeatureFlag::mark_prompt) {
         return false;
     }
-    write_to_output!(out, "\x1b]133;A;click_events=1\x07");
+    static TEST_BALLOON: OnceCell<()> = OnceCell::new();
+    if TEST_BALLOON.set(()).is_ok() {
+        write_to_output!(out, "\x1b]133;A;click_events=1\x1b\\");
+    } else {
+        write_to_output!(out, "\x1b]133;A;click_events=1\x07");
+    }
     true
 }