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
<ESC>[4:5m  # this is a dashed 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

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.