tmux: add utf8 terminal-features

Nicholas,

as discussed / suggested in https://github.com/tmux/tmux/issues/5546 I had
tried to implement utf8 in terminal-features which seems quite trivial.

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	30 Aug 2026 15:03:51 -0000
@@ -14,7 +14,7 @@
 .\" IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
 .\" OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: August 25 2026 $
+.Dd $Mdocdate: August 30 2026 $
 .Dt TMUX 1
 .Os
 .Sh NAME
@@ -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	30 Aug 2026 15:06:47 -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. */
@@ -476,7 +484,8 @@ 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 +533,8 @@ 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);


-- 
wbr, Kirill