Whitelist some incorrectly labeled "color" fonts under Xft

* etc/PROBLEMS: Document problem with fonts incorrectly labeled
as color fonts under Xft.

* src/ftfont.c (xft_color_font_whitelisted_p): New function.
(ftfont_spec_pattern, ftfont_list): Respect whitelisting of
"color" fonts under Xft.
* src/xftfont.c (syms_of_xftfont): New variable
`xft-color-font-whitelist'.

1 files changed, 16 insertions, 0 deletions

diff --git a/etc/PROBLEMS b/etc/PROBLEMS
index 3e0b73a4d59..4224171298e 100644
--- a/etc/PROBLEMS
+++ b/etc/PROBLEMS
@@ -805,6 +805,22 @@ to take advantage of.
 
 * Runtime problems related to font handling
 
+** Some fonts are detected but not usable under Xft.
+
+Some fonts might not be usable under Emacs even though they show up in
+the font family list when Emacs is built with Xft.  This is because
+Emacs prevents fonts that have color glyphs (such as color Emoji) from
+being used, since they typically cause Xft crashes.
+
+On some GNU/Linux systems, fonts (such as Source Code Pro) that do not
+have color glyphs are reported as color fonts, causing them to be
+unavailable when using Xft.  This is known to happen under Fedora
+GNU/Linux 36 or later, and possibly other distributions as well.
+
+If you encounter a such a font, you can enable it while ignoring other
+fonts that actually have color glyphs by adding its family name to the
+list `xft-color-font-whitelist'.
+
 ** Characters are displayed as empty boxes or with wrong font under X.
 
 *** This may be due to your local fontconfig customization.