Look at COLORFGBG for automatic background again (improved)

[blueyed]

Commit 3baba1e removed the feature where Vim looks at $COLORFGBG to auto-set the background to light/dark.

/*
 * Return "dark" or "light" depending on the kind of terminal.
 * This is just guessing!  Recognized are:
 * "linux"         Linux console
 * "screen.linux"   Linux console with screen
 * "cygwin"        Cygwin shell
 * "putty"         Putty program
 * We also check the COLORFGBG environment variable, which is set by
 * rxvt and derivatives. This variable contains either two or three
 * values separated by semicolons; we want the last value in either
 * case. If this value is 0-6 or 8, our background is dark.
 */
static char_u *term_bg_default(void)
{
  char_u      *p;

  if (STRCMP(T_NAME, "linux") == 0
      || STRCMP(T_NAME, "screen.linux") == 0
      || STRCMP(T_NAME, "cygwin") == 0
      || STRCMP(T_NAME, "putty") == 0
      || ((p = (char_u *)os_getenv("COLORFGBG")) != NULL
          && (p = vim_strrchr(p, ';')) != NULL
          && ((p[1] >= '0' && p[1] <= '6') || p[1] == '8')
          && p[2] == NUL))
    return (char_u *)"dark";
  return (char_u *)"light";
}

It would be nice to re-add this, but improve it (if possible), so that the
real value of a color would be used, i.e. depending on the terminal palette.

[blueyed]

This should probably also try to query the terminals background color, e.g. for urxvt via printf '\e]11;?\e\\':

% stty -icanon -echo min 0 time 1 ; printf '\e]11;?\e\\'; read bg
% echo $bg | cat -A
^[]11;rgb:0000/2b00/3600^[$

[egmontkob]

See also https://bugzilla.gnome.org/show_bug.cgi?id=733423

[justinmk]

Looks like urxvt and gnome-terminal support the "\x1b]11;?\x07" request, so there's no need to implement $COLORFGBG scraping.

Closed by #9509