Add a new escape code to allow terminal programs to trigger desktop notifications

Fixes #1474

docs/protocol-extensions.rst

@@ -228,3 +228,126 @@ no ill-effects in other terminal emulators.
 In case you're using software that can't be easily adapted to this
 protocol extension, it can be disabled by specifying ``no-append`` to the
 :opt:`clipboard_control` setting.
+
+
+.. _desktop_notifications:
+
+
+Desktop notifications
+---------------------------------
+
+|kitty| implements an extensible escape code (OSC 99) to show desktop
+notifications. It is easy to use from shell scripts and fully extensible to
+show title and body.  Clicking on the notification can optionally focus the
+window it came from, and/or send an escape code back to the application running
+in that window.
+
+The design of the escape code is partially based on the discussion in
+the defunct
+`terminal-wg <https://gitlab.freedesktop.org/terminal-wg/specifications/-/issues/13>`_
+
+The escape code has the form::
+
+    <OSC> 99 ; metadata ; payload <terminator>
+
+Here ``<OSC>`` is :code:`<ESC>]` and ``<terminator>`` is :code:`<ESC><backslash>`.  The
+metadata is a section of colon separated :code:`key=value` pairs. Every key
+must be a single character from the set :code:`[a-zA-Z]` and every value must
+be a character from the set :code:`a-zA-Z0-9-_/\+.,(){}[]*&^%$#@!`~`. The
+payload must be interpreted based on the metadata section. The two semi-colons
+*must* always be present even when no metadata is present.
+
+Before going into details, lets see how one can display a simple, single line
+notification from a shell script::
+
+    printf '\x1b]99;;Hello world\x1b\\'
+
+To show a message with a title and a body::
+
+    printf '\x1b]99;i=1:d=0;Hello world\x1b\\'
+    printf '\x1b]99;i=1:d=1:p=body;This is cool\x1b\\'
+
+The most important key in the metadata is the ``p`` key, it controls how the
+payload is interpreted. A value of ``title`` means the payload is setting the
+title for the notification. A value of ``body`` means it is setting the body,
+and so on, see the table below for full details.
+
+The design of the escape code is fundamentally chunked, this is because
+different terminal emulators have different limits on how large a single escape
+code can be. Chunking is accomplished by the ``i`` and ``d`` keys. The ``i``
+key is the *notification id* which can be any string containing the characters
+``[a-zA-Z0-9_-+.]``. The ``d`` key stands for *done* and
+can only take the values ``0`` and ``1``. A value of ``0`` means the
+notification is not yet done and the terminal emulator should hold off
+displaying it. A value of ``1`` means the notification is done, and should be
+displayed. You can specify the title or body multiple times and the terminal
+emulator will concatenate them, thereby allowing arbitrarily long text
+(terminal emulators are free to impose a sensible limit to avoid
+Denial-of-Service attacks).
+
+Both the ``title`` and ``body`` payloads must be either UTF-8 encoded plain
+text with no embedded escape codes, or UTF-8 text that is base64 encoded, in
+which case there must be an ``e=1`` key in the metadata to indicate the payload
+is base64 encoded.
+
+When the user clicks the notification, a couple of things can happen, the
+terminal emulator can focus the window from which the notification came, and/or
+it can send back an escape code to the application indicating the notification
+was activated. This is controlled by the ``a`` key which takes a comma
+separated set of values, ``report`` and ``focus``. The value ``focus`` means
+focus the window from which the notification was issued and is the default.
+``report`` means send an escape code back to the application. The format of the
+escape code is::
+
+    <OSC> 99 ; i=identifier ; <terminator>
+
+The value of ``identifier`` comes from the ``i`` key in the escape code sent by
+the application. If the application sends no identifier, then the terminal
+*must* use ``i=0``. Actions can be preceded by a negative sign to turn them
+off, so for example if you do not want any action, turn off the default
+``focus`` action with::
+
+    a=-focus
+
+Complete specification of all the metadata keys is in the table below. If a
+terminal emulator encounters a key in the metadata it does not understand,
+the key *must* be ignored, to allow for future extensibility of this escape
+code. Similarly if values for known keys are unknown, the terminal emulator
+*should* either ignore the entire escape code or perform a best guess effort
+to display it based on what it does understand.
+
+.. note::
+   It is possible to extend this escape code to allow specifying an icon for
+   the notification, however, given that some platforms, such as macOS, dont
+   allow displaying custom icons on a notification, at all, it was decided to
+   leave it out of the spec for the time being.
+
+   Similarly, features such a scheduled notifications could be added in future
+   revisions.
+
+
+=======  ====================  =========  =================
+Key      Value                 Default    Description
+=======  ====================  =========  =================
+``a``    Comma separated list  ``focus``  What action to perform when the
+         of ``report``,                   notification is clicked
+         ``focus``, with
+         optional leading
+         ``-``
+
+``d``    ``0`` or ``1``        ``1``      Indicates if the notification is
+                                          complete or not.
+
+``e``    ``0`` or ``1``        ``0``      If set to ``1`` means the payload is base64 encoded UTF-8,
+                                          otherwise it is plain UTF-8 text with no C0 control codes in it
+
+``i``    ``[a-zA-Z0-9-_+.]``   ``0``      Identifier for the notification
+
+``p``    One of ``title`` or   ``title``  Whether the payload is the notification title or body. If a
+         ``body``.                        notification has no title, the body will be used as title.
+=======  ====================  =========  =================
+
+
+.. note::
+   |kitty| also supports the legacy OSC 9 protocol developed by iTerm2 for
+   desktop notifications.