kitty/docs/underlines.rst

Colored and styled underlines
================================

|kitty| supports colored and styled (wavy) underlines. This is of particular use
in terminal based text editors such as :program:`vim` and :program:`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 <https://en.wikipedia.org/wiki/ANSI_escape_code#CSI_(Control_Sequence_Introducer)_sequences>`__)

To set the underline style::

    <ESC>[4:0m  # no underline
    <ESC>[4:1m  # straight underline
    <ESC>[4:2m  # double underline
    <ESC>[4:3m  # curly underline
    <ESC>[4:4m  # dotted underline
    <ESC>[4:5m  # dashed underline
    <ESC>[4m    # straight underline (for backwards compat)
    <ESC>[24m   # 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

The underline color must remain the same under reverse video, if it has a color,
if not, it should follow the foreground color.

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