bug#75024: [PATCH] Fix check for underlining capability on ttys

[Gerd Möllmann]

Tags: patch

With current master

emacs -nw -Q on Terminal.app, $TERM=xterm-256color

1. (display-supports-face-attributes-p '(underline t))
     => nil

2. C-h f context-menu-map RET
   => The separator line in *Help* in underlined, which
   means that term.c thinks that underlines can be used.

display-supports-face-attribute-p uses tty_capable_p in term.c.
This code in tty_capable_p looks wrong:

  TTY_CAPABLE_P_TRY (tty,
                     TTY_CAP_UNDERLINE,   tty->TS_enter_underline_mode,
                     NC_UNDERLINE);
  TTY_CAPABLE_P_TRY (tty,
                     TTY_CAP_UNDERLINE_STYLED,    tty->TF_set_underline_style,
                     NC_UNDERLINE);

It returns false as soon as it finds TS_enter_underline_mode is cannot
be used, and doesn't check TS_set_underline_style. The output code uses
one or the other

  if (face->underline && MAY_USE_WITH_COLORS_P (tty, NC_UNDERLINE))
    {
      if (face->underline == FACE_UNDERLINE_SINGLE
          || !tty->TF_set_underline_style)
        OUTPUT1_IF (tty, tty->TS_enter_underline_mode);
      else if (tty->TF_set_underline_style)
        {
          char *p;
          p = tparam (tty->TF_set_underline_style, NULL, 0, face->underline, 0, 
0, 0);
          OUTPUT (tty, p);
          xfree (p);
        }
    }

In GNU Emacs 31.0.50 (build 6, aarch64-apple-darwin24.2.0) of 2024-12-22
 built on pro2
Repository revision: d481da70010eab163d12f770ed11f8fef171406a
Repository branch: cl-packages
System Description:  macOS 15.2

Configured using:
 'configure --without-ns --cache-file
 /var/folders/1d/k_6t25f94sl83szqbf8gpkrh0000gn/T//config.cache.cl-packages
 --with-native-compilation --with-mps=yes CC=clang
 'CFLAGS=-Wgnu-imaginary-constant -Wunused-result -g
 -fno-omit-frame-pointer -F
 
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks
 -Wno-ignored-attributes -Wno-flag-enum -Wno-missing-method-return-type
 -Wno-variadic-macros -Wno-strict-prototypes -Wno-availability
 -Wno-nullability-completeness' --prefix=/Users/gerd/.local'

0001-Fix-check-for-underlining-capability-on-ttys.patch
Description: Text Data