Check the terminfo(5) U8 capability and disable using UTF-8 for ACS if

it is present and zero. This is useful for users with terminals or fonts
that do not correctly support UTF-8 line drawing characters. GitHub
issue 927, reported by Hiroaki Yamazoe and Akinori Hattori.

tmux.h

@@ -401,6 +401,7 @@ enum tty_code_code {
 	TTYC_SS,	/* set cursor style, Ss */
 	TTYC_TC,	/* 24-bit "true" colour, Tc */
 	TTYC_TSL,	/* to_status_line, tsl */
+	TTYC_U8,
 	TTYC_VPA,	/* row_address, cv */
 	TTYC_XENL,	/* eat_newline_glitch, xn */
 	TTYC_XT,	/* xterm(1)-compatible title, XT */
@@ -1701,6 +1702,7 @@ int		 tty_term_flag(struct tty_term *, enum tty_code_code);
 const char	*tty_term_describe(struct tty_term *, enum tty_code_code);
 
 /* tty-acs.c */
+int		 tty_acs_needed(struct tty *);
 const char	*tty_acs_get(struct tty *, u_char);
 
 /* tty-keys.c */

tty-acs.c

@@ -74,23 +74,48 @@ tty_acs_cmp(const void *key, const void *value)
 	return (ch - entry->key);
 }
 
+/* Should this terminal use ACS instead of UTF-8 line drawing? */
+int
+tty_acs_needed(struct tty *tty)
+{
+	if (tty == NULL)
+		return (0);
+
+	/*
+	 * If the U8 flag is present, it marks whether a terminal supports
+	 * UTF-8 and ACS together.
+	 *
+	 * If it is present and zero, we force ACS - this gives users a way to
+	 * turn off UTF-8 line drawing.
+	 *
+	 * If it is nonzero, we can fall through to the default and use UTF-8
+	 * line drawing on UTF-8 terminals.
+	 */
+	if (tty_term_has(tty->term, TTYC_U8) &&
+	    tty_term_number(tty->term, TTYC_U8) == 0)
+		return (1);
+
+	if (tty->flags & TTY_UTF8)
+		return (0);
+	return (1);
+}
+
 /* Retrieve ACS to output as a string. */
 const char *
 tty_acs_get(struct tty *tty, u_char ch)
 {
 	struct tty_acs_entry	*entry;
 
-	/* If not a UTF-8 terminal, use the ACS set. */
+	/* Use the ACS set instead of UTF-8 if needed. */
-	if (tty != NULL && !(tty->flags & TTY_UTF8)) {
+	if (tty_acs_needed(tty)) {
 		if (tty->term->acs[ch][0] == '\0')
 			return (NULL);
 		return (&tty->term->acs[ch][0]);
 	}
 
 	/* Otherwise look up the UTF-8 translation. */
-	entry = bsearch(&ch,
+	entry = bsearch(&ch, tty_acs_table, nitems(tty_acs_table),
-	    tty_acs_table, nitems(tty_acs_table), sizeof tty_acs_table[0],
+	    sizeof tty_acs_table[0], tty_acs_cmp);
-	    tty_acs_cmp);
 	if (entry == NULL)
 		return (NULL);
 	return (entry->string);

tty-term.c

@@ -254,6 +254,7 @@ static const struct tty_term_code_entry tty_term_codes[] = {
 	[TTYC_TC] = { TTYCODE_FLAG, "Tc" },
 	[TTYC_TSL] = { TTYCODE_STRING, "tsl" },
 	[TTYC_VPA] = { TTYCODE_STRING, "vpa" },
+	[TTYC_U8] = { TTYCODE_NUMBER, "U8" },
 	[TTYC_XENL] = { TTYCODE_FLAG, "xenl" },
 	[TTYC_XT] = { TTYCODE_FLAG, "XT" },
 };

tty.c

@@ -71,8 +71,6 @@ static void	tty_default_colours(struct grid_cell *,
 static void	tty_default_attributes(struct tty *, const struct window_pane *,
 		    u_int);
 
-#define tty_use_acs(tty) \
-	(tty_term_has((tty)->term, TTYC_ACSC) && !((tty)->flags & TTY_UTF8))
 #define tty_use_margin(tty) \
 	((tty)->term_type == TTY_VT420)
 
@@ -278,6 +276,7 @@ tty_open(struct tty *tty, char **cause)
 void
 tty_start_tty(struct tty *tty)
 {
+	struct client	*c = tty->client;
 	struct termios	 tio;
 
 	if (tty->fd != -1 && tcgetattr(tty->fd, &tty->tio) == 0) {
@@ -299,10 +298,14 @@ tty_start_tty(struct tty *tty)
 	tty_putcode(tty, TTYC_SMCUP);
 
 	tty_putcode(tty, TTYC_SMKX);
-	if (tty_use_acs(tty))
-		tty_putcode(tty, TTYC_ENACS);
 	tty_putcode(tty, TTYC_CLEAR);
 
+	if (tty_acs_needed(tty)) {
+		log_debug("%s: using capabilities for ACS", c->name);
+		tty_putcode(tty, TTYC_ENACS);
+	} else
+		log_debug("%s: using UTF-8 for ACS", c->name);
+
 	tty_putcode(tty, TTYC_CNORM);
 	if (tty_term_has(tty->term, TTYC_KMOUS))
 		tty_puts(tty, "\033[?1000l\033[?1002l\033[?1006l\033[?1005l");
@@ -351,7 +354,7 @@ tty_stop_tty(struct tty *tty)
 		return;
 
 	tty_raw(tty, tty_term_string2(tty->term, TTYC_CSR, 0, ws.ws_row - 1));
-	if (tty_use_acs(tty))
+	if (tty_acs_needed(tty))
 		tty_raw(tty, tty_term_string(tty->term, TTYC_RMACS));
 	tty_raw(tty, tty_term_string(tty->term, TTYC_SGR0));
 	tty_raw(tty, tty_term_string(tty->term, TTYC_RMKX));
@@ -1417,7 +1420,7 @@ tty_reset(struct tty *tty)
 	struct grid_cell	*gc = &tty->cell;
 
 	if (!grid_cells_equal(gc, &grid_default_cell)) {
-		if ((gc->attr & GRID_ATTR_CHARSET) && tty_use_acs(tty))
+		if ((gc->attr & GRID_ATTR_CHARSET) && tty_acs_needed(tty))
 			tty_putcode(tty, TTYC_RMACS);
 		tty_putcode(tty, TTYC_SGR0);
 		memcpy(gc, &grid_default_cell, sizeof *gc);
@@ -1767,7 +1770,7 @@ tty_attributes(struct tty *tty, const struct grid_cell *gc,
 		tty_putcode(tty, TTYC_INVIS);
 	if (changed & GRID_ATTR_STRIKETHROUGH)
 		tty_putcode(tty, TTYC_SMXX);
-	if ((changed & GRID_ATTR_CHARSET) && tty_use_acs(tty))
+	if ((changed & GRID_ATTR_CHARSET) && tty_acs_needed(tty))
 		tty_putcode(tty, TTYC_SMACS);
 }