Extensions to the xterm protocol

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.

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 (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>[4:4m  # this is a dotted underline (not implemented in kitty)
<ESC>[4:5m  # this is a dashed underline (not implemented in kitty)
<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

To detect support for this feature in a terminal emulator, query the terminfo database for the Su boolean capability.

Graphics rendering

See The terminal 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 reliably 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: 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 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. Note that this means there are no repeat and release events for these keys and also for the left and right shift 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

Schematically:

<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 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, 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 this discussion and this FAQ 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

Saving and restoring the default foreground/background/selection/cursor colors

It is often useful for a full screen application with its own color themes to set the default foreground, background, selection and cursor colors. This allows for various performance optimizations when drawing the screen. The problem is that if the user previously used the escape codes to change these colors herself, then running the full screen application will lose her changes even after it exits. To avoid this, kitty introduces a new pair of OSC escape codes to push and pop the current color values from a stack:

<ESC>]30001<ESC>\  # push onto stack
<ESC>]30101<ESC>\  # pop from stack

These escape codes save/restore the so called dynamic colors, default background, default foreground, selection background, selection foreground and cursor color.

Pasting to clipboard

kitty implements the OSC 52 escape code protocol to get/set the clipboard contents (controlled via the clipboard_control setting). There is one difference in kitty's implementation compared to some other terminal emulators. kitty allows sending arbitrary amounts of text to the clipboard. It does so by modifying the protocol slightly. Successive OSC 52 escape codes to set the clipboard will concatenate, so:

<ESC>]52;c;<payload1><ESC>\
<ESC>]52;c;<payload2><ESC>\

will result in the clipboard having the contents payload1 + payload2. To send a new string to the clipboard send an OSC 52 sequence with an invalid payload first, for example:

<ESC>]52;c;!<ESC>\

Here ! is not valid base64 encoded text, so it clears the clipboard.