`COLORFGBG`

[tautropfli]

I don't particularly like the COLORFGBG mechanism, but it seems to be useful to people who use a terminal that doesn't support querying. See dalance/termbg#18 (comment).

Terminals

• urxvt(1): https://man.archlinux.org/man/extra/rxvt-unicode/urxvt.1.en#COLORFGBG
• Discussion in alacritty: https://www.github.com/alacritty/alacritty/issues/3996
• Konsole sets COLORFGBG but only communicates dark-on-light vs light-on-dark: https://invent.kde.org/utilities/konsole/-/blob/f62bce57d00bcc5fc7bfb265303bd3bccaef2c05/src/session/Session.cpp#L557-L561
• iTerm sets it too, falls back to 15;0 and 0;15 (light-on-dark / dark-on-light) if no colors match: https://gitlab.com/gnachman/iterm2/-/blob/66dae7174eea71d289bce06b0513a35b8fcc857e/sources/iTermAdvancedSettingsModel.m#L377

Applications

• https://github.com/dalance/termbg
• https://github.com/rocky/shell-term-background and https://unix.stackexchange.com/questions/245378/common-environment-variable-to-set-dark-or-light-terminal-background
• ncurses: https://github.com/ThomasDickey/ncurses-snapshots/blob/269325de08c2f7a3088ecc9f515933301c8d99a6/ncurses/base/lib_set_term.c#L509. ncurses also has its own NCURSES_ASSUMED_COLORS env var to communicate color fg / bg.
• grief: https://github.com/adamyg/grief/blob/5401f2f184025e8ac1806ab7de62793020831b2a/gr/ttycmd.c#L61
• vim: https://github.com/vim/vim/blob/a6d9e3c4e07f73f6523699961d8b7b54bdb80cf6/src/term.c#L3227
• neovim notably doesn't support it

Notes

• Most terminals listed above that set COLORFGBG also support the corresponding escape sequences, so preferring COLORFGBG would result in incorrect values when the values are changed at runtime (the escape sequences get the up-to-date value while the env var does not change).
• Neovim doesn't see a need for supporting COLORFGBG

[tautropfli]

I think documenting my stance in the crate docs should do for now.