The multiple cursors protocol - kitty
Description: Many editors support something called multiple cursors in which you can make the same changes at multiple locations in a file and the editor shows you cursors at each of the locations. In a termina...

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# The multiple cursors protocol

Added in version 0.43.0.

Many editors support something called multiple cursors in which you can make the same changes at multiple locations in a file and the editor shows you cursors at each of the locations. In a terminal context editors typically implement this by showing some Unicode glyph at each location instead of the actual cursor. This is sub-optimal since actual cursors implemented by the terminal have many niceties like smooth animation [anim], auto adjust colors [rv], etc. To address this and other use cases, this protocol allows terminal programs to request that the terminal display multiple cursors at specific locations on the screen.

## Quickstart

An example, showing how to use the protocol:

    # Show cursors of the same shape as the main cursor at y=4, x=5
    printf "\e[>29;2:4:5 q"
    # Show more cursors on the seventh line, of various shapes, the underline shape is shown twice
    printf "\e[>1;2:7:1 q\e[>2;2:7:3 q\e[>3;2:7:5;2:7:7 q"

The escape code to show a cursor has the following structure (ignore spaces they are present for readability only):

    CSI > SHAPE;CO-ORD TYPE : CO-ORDINATES ; CO-ORD TYPE : CO-ORDINATES ... TRAILER

Here `CSI` is the two bytes ESC (`0x1b`) and [ (`0x5b`). `SHAPE` can be one of:

- `0`: No cursor

- `1`: Block cursor

- `2`: Beam cursor

- `3`: Underline cursor

- `29`: Follow the shape of the main cursor

- `30`: Change the color of text under extra cursors

- `40`: Change the color of extra cursors

- `100`: Used for querying currently set cursors

`CO-ORD TYPE` can be one of:

- `0`: This refers to the position of the main cursor and has no following co-ordinates.

- `2`: In this case the following co-ordinates are pairs of numbers pointing to cells in the form `y:x` with the origin in the top left corner at `1,1`. There can be any number of pairs, the terminal must treat each pair as a new location to set a cursor.

- `4`: In this case the following co-ordinates are sets of four numbers that define a rectangle in the same co-ordinate system as above of the form: `top:left:bottom:right`. The shape is set on every cell in the rectangle from the top left cell to the bottom right cell, inclusive. If no numbers are provided, the rectangle is the full screen. There can be any number of rectangles, the terminal must treat each set of four numbers as a new rectangle.

The sequence of `CO-ORD TYPE : CO-ORDINATES` can be repeated any number of times separated by `;`. The `SHAPE` will be set on the cells indicated by each such group. For example: `-1;2:3:4;4:5:6:7:8` will set the shape `-1` at the cell `(3, 2)` and in the rectangle `(6, 5)` to `(8, 7)` inclusive.

Finally, the `TRAILER` terminates the sequence and is the bytes SPACE (`0x20`) and q (`0x71`).

Terminals must ignore cells that fall outside the screen. That means, for rectangle co-ordinates only the intersection of the rectangle with the screen must be considered, and point co-ordinates that fall outside of the screen are simply ignored, with no effect.

Terminals must ignore extra co-ordinates, that means if an odd number of co-ordinates are specified for type `2` the last co-ordinate is ignored. Similarly for type `4` if the number of co-ordinates is not a multiple of four, the last `1 <= n <= 3` co-ordinates are ignored, as if they were not specified.

## Querying for support

A terminal program can query the terminal emulator for support of this protocol by sending the escape code:

    CSI > TRAILER

In this case a supporting terminal must reply with:

    CSI > 1;2;3;29;30;40;100;101 TRAILER

Here, the list of numbers indicates the cursor shapes and other operations the terminal supports and can be any subset of the above. No numbers indicates the protocol is not supported. To avoid having to wait with a timeout for a response from the terminal, the client should send this query code immediately followed by a request for the primary device attributes. If the terminal responds with an answer for the device attributes without an answer for the query the terminal emulator does not support this protocol at all.

Terminals must respond to these queries in FIFO order, so that multiplexers that split a single screen know which split to send responses too.

## Clearing previously set multi-cursors

The cursor at a cell is cleared by setting its shape to `0`. The most common operation is to clear all previously set multi-cursors. This is easily done using the rectangle co-ordinate system above, like this:

    CSI > 0;4 TRAILER

For more precise control different co-ordinate types can be used. This is particularly important for multiplexers that split up the screen and therefore need to re-write these escape codes.

## Changing the color of extra cursors

In order to visually distinguish extra cursors from the main cursor, it is possible to specify a color pair for extra cursors. Note that for performance reasons, there is only a single color pair that all extra cursors share. The color pair consists of the cursor color and the color for text in the cell the cursor is on.

To change this color pair use an escape code of the form:

    CSI > WHICH ; COLOR_SPACE : COLOR_PARAMETER1 : COLOR_PARAMETER2 : ... TRAILER

Here, `WHICH` is `30` to set the color of text under the cursor and `40` to set the color of the cursor itself (these numbers mimic the SGR codes for foreground and background respectively).

The `COLOR_SPACE` parameter sets the type of color, it can take values:

`0` - unset color is same as for main cursor. No color parameters. `1` - special which typically means some kind of reverse video effect, see below `2` - sRGB color, with three color parameters, red, green and blue as numbers from 0 to 255 `5` - Indexed color with one color parameter which is an index into the color table from 0 to 255

When the cursor color is set to special via `40` it means the block cursor must be rendered with a reverse video effect where the cursor color becomes the foreground color of the cell under the cursor and the foreground color of the cell becomes its background color. Implementations are free to adjust these colors to ensure suitable contrast levels. In this case the text color set by `30` must be ignored.

When the cursor color is not set to special but the text color via `30` is set to special, then that means the foreground color of the cell with the cursor must be changed to its background color for a partial reverse video effect.

When unset, aka, set to `0` the cursors must be the same color as the main cursor. In particular if the main color is using a reverse video effect, the extra cursors must use the exact same colors as the main cursor, not the colors of the cells they are on.

## Querying for already set cursors

Programs can ask the terminal what extra cursors are currently set, by sending the escape code:

    CSI > 100 TRAILER

The terminal must respond with one escape code:

    CSI > 100; SHAPE:CO-ORDINATE TYPE:CO-ORDINATES ; ... TRAILER

Here, the `SHAPE:CO-ORDINATE TYPE:CO-ORDINATES` block can be repeated any number of times, separated by `;`. This response gives the set of shapes and positions currently active. If no cursors are currently active, there will be no blocks, just an empty response of the form:

    CSI > 100 TRAILER

Again, terminals must respond in FIFO order so that multiplexers know where to direct the responses.

## Querying for extra cursor colors

Programs can ask the terminal what cursor colors are currently set, by sending escape code:

    CSI > 101 TRAILER

The terminal must respond with one escape code:

    CSI > 101 ; 30 : COLOR_SPACE : COLOR_PARAMETERS ; 40 : COLOR_SPACE : COLOR_PARAMETERS TRAILER

The number and type of `COLOR_PARAMETERS` depends on the preceding `COLOR_SPACE` and can be omitted for some `COLOR_SPACE` values. See the section Changing the color of extra cursors for details.

## Interaction with other terminal controls and state

The main cursor

  The extra cursors must all have the same color and opacity and blink state as the main cursor. The main cursor’s visibility must not affect the visibility of the extra cursors. Their visibility and shape are controlled only by this protocol.

Clearing the screen

  The escape codes used to clear the screen (ED) with parameters 2, 3 and 22 must remove all extra cursors, this is so that the clear command can be used by users to clear the screen of extra cursors.

Reset*

  This must remove all extra cursors.

Alternate screen*

  Switching between the main and alternate screens must remove all extra cursors.

Scrolling

  The index (IND) and reverse index (RI) escape codes that cause screen contents to scroll into scrollback or off screen must not affect the extra cursors in any way. They remain at exactly the same position. It is up to applications to manage extra cursor positions when using these escape codes if needed. There are not a lot of use cases for scrolling extra cursors with screen content, since extra cursors are meant to be ephemeral and on screen only, not in scrollback. This allows terminals to avoid the extra overhead of adjusting positions of the extra cursors on every scroll.

## Footnotes

[anim]

kitty allows the cursor blink to be `animated` using any CSS easing function. This cannot be implemented using fake cursors.

[rv]

kitty has a special “reverse video” color mode for cursors where the color of the cursor and the text under the cursor is adjusted based on the color of the cell under the cursor. This also cannot be implemented using fake cursors.