On Mon, 31 Aug 2026 10:16:28 +0200,
Nicholas Marriott <nicholas.marriott@gmail.com> wrote:
> 
> It is a little unfortunate UTF8 is a client flag rather than tty_term but
> it kind of has to be, so this looks the right approach.
> 
> I think tty_feature_present should special-case this as well (you can
> check term->tty->client->flags) and then you need this in window-client.c
> also so it shows up in the info pane (C-b D i):
> 
> --- window-client.c     5 Aug 2026 08:54:56 -0000       1.48
> +++ window-client.c     31 Aug 2026 08:13:37 -0000
> @@ -102,7 +102,8 @@ static const char *window_client_info_li
>         WINDOW_CLIENT_FEATURE(sync) " "
>         WINDOW_CLIENT_FEATURE(title),
>         "              #[#{E:tree-mode-border-style},acs]x#[default] "
> -       WINDOW_CLIENT_FEATURE(usstyle),
> +       WINDOW_CLIENT_FEATURE(usstyle) " "
> +       WINDOW_CLIENT_FEATURE(utf8),
> 
> "#[#{E:tree-mode-border-style},acs]qqqqqqqqqqqqqqn#{R:q,#{window_width}}#[default]",
> 
>         "#[fg=themelightgrey]prefix
>  #[#{E:tree-mode-border-style},acs]x#[default] "
> 

Thanks, here updated diff which should report utf8 state when it is enabled
via locale, -u or the new feature.

Ok?

Index: tmux.1
===================================================================
RCS file: /home/cvs/src/usr.bin/tmux/tmux.1,v
diff -u -p -r1.1159 tmux.1
--- tmux.1	25 Aug 2026 08:37:08 -0000	1.1159
+++ tmux.1	31 Aug 2026 10:45:16 -0000
@@ -5111,6 +5111,8 @@ Supports
 title setting.
 .It usstyle
 Allows underscore style and colour to be set.
+.It utf8
+Supports UTF\-8 output.
 .El
 .It Ic terminal\-overrides[] Ar string
 Allow terminal descriptions read using
Index: tty-features.c
===================================================================
RCS file: /home/cvs/src/usr.bin/tmux/tty-features.c,v
diff -u -p -r1.42 tty-features.c
--- tty-features.c	17 Aug 2026 14:47:41 -0000	1.42
+++ tty-features.c	31 Aug 2026 10:42:21 -0000
@@ -358,6 +358,13 @@ static const struct tty_feature tty_feat
 	0
 };
 
+/* Terminal supports UTF-8. */
+static const struct tty_feature tty_feature_utf8 = {
+	"utf8",
+	NULL,
+	0
+};
+
 /* Available terminal features. */
 static const struct tty_feature *const tty_features[] = {
 	&tty_feature_256,
@@ -380,7 +387,8 @@ static const struct tty_feature *const t
 	&tty_feature_strikethrough,
 	&tty_feature_sync,
 	&tty_feature_title,
-	&tty_feature_usstyle
+	&tty_feature_usstyle,
+	&tty_feature_utf8
 };
 
 /* Parse features for client. */
@@ -463,6 +471,9 @@ tty_feature_present(struct tty_term *ter
 	u_int				 i;
 	char				*copy;
 
+	if (strcmp(name, "utf8") == 0)
+		return ((term->tty->client->flags & CLIENT_UTF8) != 0);
+
 	for (i = 0; i < nitems(tty_features); i++) {
 		tf = tty_features[i];
 		if (strcmp(tf->name, name) == 0) {
@@ -476,7 +487,8 @@ tty_feature_present(struct tty_term *ter
 	 * We don't just have the feature flag set. Check if the capabilities
 	 * supported by the client are actual set instead.
 	 */
-	if (tf == NULL || strcmp(name, "ignorefkeys") == 0)
+	if (tf == NULL || tf->capabilities == NULL ||
+	    strcmp(name, "ignorefkeys") == 0)
 		return (0);
 	if (tf->flags != 0 && (term->flags & tf->flags) != tf->flags)
 		return (0);
@@ -524,6 +536,8 @@ tty_apply_features(struct tty_term *term
 			}
 		}
 		term->flags |= tf->flags;
+		if (tf == &tty_feature_utf8)
+			c->flags |= CLIENT_UTF8;
 	}
 	if ((term->applied_features|feat) == term->applied_features)
 		return (0);
Index: window-client.c
===================================================================
RCS file: /home/cvs/src/usr.bin/tmux/window-client.c,v
diff -u -p -r1.48 window-client.c
--- window-client.c	5 Aug 2026 08:54:56 -0000	1.48
+++ window-client.c	31 Aug 2026 10:42:21 -0000
@@ -102,7 +102,8 @@ static const char *window_client_info_li
 	WINDOW_CLIENT_FEATURE(sync) " "
 	WINDOW_CLIENT_FEATURE(title),
 	"              #[#{E:tree-mode-border-style},acs]x#[default] "
-	WINDOW_CLIENT_FEATURE(usstyle),
+	WINDOW_CLIENT_FEATURE(usstyle) " "
+	WINDOW_CLIENT_FEATURE(utf8),
 	"#[#{E:tree-mode-border-style},acs]qqqqqqqqqqqqqqn#{R:q,#{window_width}}#[default]",
 
 	"#[fg=themelightgrey]prefix        #[#{E:tree-mode-border-style},acs]x#[default] "


-- 
wbr, Kirill