= Extensions to the xterm protocol
:toc:
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kitty has a few extensions to the xterm protocol, to enable advanced features.
These are typically in the form of new or re-purposed escape codes. While these
extensions are currently kitty specific, it would be nice to get some of them
adopted more broadly, to push the state of terminal emulators forward.

The goal of these extensions is to be as small an unobtrusive as possible,
while filling in some gaps in the existing xterm protocol. In particular, one
of the goals of this specification is explicitly not to "re-imagine" the tty.
The tty should remain what it is -- a device for efficiently processing text
received as a simple byte stream. Another objective is to only move the minimum
possible amount of extra functionality into the terminal program itself. This
is to make it as easy to implement these protocol extensions as possible,
thereby hopefully encouraging their widespread adoption.

If you wish to discuss these extensions, propose additions/changes to them
please do so by opening issues in the github bug tracker.

toc::[]

== Colored and styled underlines

kitty supports colored and styled (wavy) underlines. This is of particular use
in terminal editors such as vim and emacs to display red, wavy underlines under
mis-spelled words and/or syntax errors. This is done by re-purposing some SGR escape codes
that are not used in modern terminals (https://en.wikipedia.org/wiki/ANSI_escape_code#CSI_codes)

To set the underline style:

```
<ESC>[4:0m  # this is no underline
<ESC>[4:1m  # this is a straight underline
<ESC>[4:2m  # this is a double underline
<ESC>[4:3m  # this is a curly underline
<ESC>[4m    # this is a straight underline (for backwards compat)
<ESC>[24m   # this is no underline (for backwards compat)
```

To set the underline color (this is reserved and as far as I can tell not actually used for anything):

```
<ESC>[58...m
```

This works exactly like the codes `38, 48` that are used to set foreground and
background color respectively.

To reset the underline color (also previously reserved and unused):

```
<ESC>[59m
```

A client can query if the terminal emulator supports this underline extension using the 
standard link:http://vt100.net/docs/vt510-rm/DECRQM[DECRQM] escape sequence, with the 
mode value `2016`, like this:

Client sends:

```
<ESC>[?2016$p
```

If the terminal supports this underline extension, it must respond with
link:http://vt100.net/docs/vt510-rm/DECRPM[DECRPM]

```
<ESC>[?2016;3$p
```


== Graphics rendering

See link:graphics-protocol.asciidoc[Graphics Protocol] for a description
of this protocol to enable drawing of arbitrary raster images in the terminal.


== Keyboard handling

There are various problems with the current state of keyboard handling. They
include:

  * No way to use modifiers other than `Ctrl` and `Alt`
  * No way to use multiple modifier keys, other than, `Shift+Alt`.
  * No way to handle different types of keyboard events, such as press, release or repeat
  * No reliable way to distinguish single `Esc` keypresses from the 
    start of a escape sequence. Currently, client programs use 
    fragile timing related hacks for this, leading to bugs, for example:
    link:https://github.com/neovim/neovim/issues/2035[neovim #2035]

There are already two distinct keyboard handling modes, _normal mode_ and
_application mode_. These modes generate different escape sequences for the
various special keys (arrow keys, function keys, home/end etc.) Most terminals
start out in normal mode, however, most shell programs like `bash` switch them to
application mode. We propose adding a third mode, named _full mode_ that addresses
the shortcomings listed above.

Switching to the new _full mode_ is accomplished using the standard private
mode DECSET escape sequence

```
<ESC>[?2017h
```

and to leave _full mode_, use DECRST

```
<ESC>[?2017l
```

The number `2017` above is not used for any existing modes, as far as I know.
Client programs can query if the terminal emulator is in _full mode_ by using 
the standard link:http://vt100.net/docs/vt510-rm/DECRQM[DECRQM] escape sequence.

The new mode works as follows:

  * All printable key presses without modifier keys are sent just as in the
    _normal mode_. This means all printable ASCII characters and in addition,
    `Enter`, `Space` and `Backspace`. Also any unicode characters generated by
    platform specific extended input modes, such as using the `AltGr` key. This
    is done so that client programs that are not aware of this mode can still
    handle basic text entry, so if a _full mode_ using program crashes and does
    not reset, the user can still issue a `reset` command in the shell to restore
    normal key handling. Note that this includes pressing the `Shift` modifier
    and printable keys.

  * For non printable keys and key combinations including one or more modifiers,
    an escape sequence encoding the key event is sent. For details on the
    escape sequence, see below.

The escape sequence encodes the following properties:

  * Type of event: `press,repeat,release`
  * Modifiers pressed at the time of the event
  * The actual key being pressed 

```
<ESC>_K<type><modifiers><key><ESC>\
```

Where `<type>` is one of `p` -- press, `r` -- release and `t` -- repeat.
Modifiers is a bitmask represented as a single base64 digit.  Shift -- `0x1`,
Alt -- `0x2`, Control -- `0x4` and Super -- `0x8`.  `<key>` is a number
(encoded in base85) corresponding to the key pressed. The key name to number
mapping is defined in link:key_encoding.asciidoc[this table].

For example:

```
<ESC>_KpGp<ESC>\  is  <Ctrl>+<Alt>+x (press)
<ESC>_KrP8<ESC>\  is  <Ctrl>+<Alt>+<Shift>+<Super>+PageUp (release)
```

This encoding means each key event is represented by 8 or 9 printable ascii
only bytes, for maximum robustness.


== Setting text styles/colors in arbitrary regions of the screen

There already exists an escape code to set *some* text attributes in arbitrary
regions of the screen,
link:https://vt100.net/docs/vt510-rm/DECCARA.html[DECCARA].  However, it is
limited to only a few attributes. kitty extends this to work with *all* SGR
attributes.  So, for example, this can be used to set the background color in
an arbitrary region of the screen. 

The motivation for this extension is the various problems with the existing
solution for erasing to background color, namely the *background color erase
(bce)* capability. See 
link:https://github.com/kovidgoyal/kitty/issues/160#issuecomment-346470545[this discussion]
for a summary of problems with *bce*. 

For example, to set the background color to blue in a
rectangular region of the screen from (3, 4) to (10, 11), you use:

```
<ESC>[2*x<ESC>[4;3;11;10;44$r<ESC>[*x
```
