Send Unicode directional isolate characters around horizontal pane

borders if the terminal support UTF-8 and an extension terminfo(5)
capability "Bidi" is present. On terminals with BiDi support (ie, VTE)
this seems to be enough to display right-to-left text acceptably enough
to be usable (with some caveats about the mouse position). Requested by
and with help from Mahmoud Elagdar in GitHub issue 2425.

grid.c

@@ -1045,14 +1045,14 @@ grid_duplicate_lines(struct grid *dst, u_int dy, struct grid *src, u_int sy,
 			    srcl->cellsize * sizeof *dstl->celldata);
 		} else
 			dstl->celldata = NULL;
-
 		if (srcl->extdsize != 0) {
 			dstl->extdsize = srcl->extdsize;
 			dstl->extddata = xreallocarray(NULL, dstl->extdsize,
 			    sizeof *dstl->extddata);
 			memcpy(dstl->extddata, srcl->extddata, dstl->extdsize *
 			    sizeof *dstl->extddata);
-		}
+		} else
+			dstl->extddata = NULL;
 
 		sy++;
 		dy++;

screen-redraw.c

@@ -32,8 +32,8 @@ static void	screen_redraw_set_context(struct client *,
 		    struct screen_redraw_ctx *);
 
 #define CELL_INSIDE 0
-#define CELL_LEFTRIGHT 1
-#define CELL_TOPBOTTOM 2
+#define CELL_TOPBOTTOM 1
+#define CELL_LEFTRIGHT 2
 #define CELL_TOPLEFT 3
 #define CELL_TOPRIGHT 4
 #define CELL_BOTTOMLEFT 5
@@ -47,6 +47,9 @@ static void	screen_redraw_set_context(struct client *,
 
 #define CELL_BORDERS " xqlkmjwvtun~"
 
+#define START_ISOLATE "\342\201\246"
+#define END_ISOLATE   "\342\201\251"
+
 static const struct utf8_data screen_redraw_double_borders[] = {
 	{ "", 0, 0, 0 },
 	{ "\342\225\221", 0, 3, 1 }, /* U+2551 */
@@ -299,7 +302,7 @@ screen_redraw_type_of_cell(struct client *c, u_int px, u_int py,
 	case 13:	/* 1101, left right bottom */
 		return (CELL_TOPJOIN);
 	case 12:	/* 1100, left right */
-		return (CELL_TOPBOTTOM);
+		return (CELL_LEFTRIGHT);
 	case 11:	/* 1011, left top bottom */
 		return (CELL_RIGHTJOIN);
 	case 10:	/* 1010, left top */
@@ -313,7 +316,7 @@ screen_redraw_type_of_cell(struct client *c, u_int px, u_int py,
 	case 5:		/* 0101, right bottom */
 		return (CELL_TOPLEFT);
 	case 3:		/* 0011, top bottom */
-		return (CELL_LEFTRIGHT);
+		return (CELL_TOPBOTTOM);
 	}
 	return (CELL_OUTSIDE);
 }
@@ -680,7 +683,7 @@ screen_redraw_draw_borders_cell(struct screen_redraw_ctx *ctx, u_int i, u_int j)
 	struct tty		*tty = &c->tty;
 	struct window_pane	*wp;
 	u_int			 cell_type, x = ctx->ox + i, y = ctx->oy + j;
-	int			 pane_status = ctx->pane_status;
+	int			 pane_status = ctx->pane_status, isolates;
 	struct grid_cell	 gc;
 	const struct grid_cell	*tmp;
 
@@ -705,11 +708,22 @@ screen_redraw_draw_borders_cell(struct screen_redraw_ctx *ctx, u_int i, u_int j)
 	}
 	screen_redraw_border_set(wp, ctx->pane_lines, cell_type, &gc);
 
+	if (cell_type == CELL_TOPBOTTOM &&
+	    (c->flags & CLIENT_UTF8) &&
+	    tty_term_has(tty->term, TTYC_BIDI))
+		isolates = 1;
+	else
+		isolates = 0;
+
 	if (ctx->statustop)
 		tty_cursor(tty, i, ctx->statuslines + j);
 	else
 		tty_cursor(tty, i, j);
+	if (isolates)
+		tty_puts(tty, END_ISOLATE);
 	tty_cell(tty, &gc, &grid_default_cell, NULL);
+	if (isolates)
+		tty_puts(tty, START_ISOLATE);
 }
 
 /* Draw the borders. */

tmux.1

@@ -5953,6 +5953,10 @@ option should be used.
 An existing extension that tells
 .Nm
 the terminal supports default colours.
+.It Em \&Bidi
+Tell
+.Nm
+that the terminal supports the VTE bidirectional text extensions.
 .It Em \&Cs , Cr
 Set the cursor colour.
 The first takes a single string argument and is used to set the colour;

tmux.h

@@ -261,6 +261,7 @@ enum tty_code_code {
 	TTYC_AX,
 	TTYC_BCE,
 	TTYC_BEL,
+	TTYC_BIDI,
 	TTYC_BLINK,
 	TTYC_BOLD,
 	TTYC_CIVIS,

tty-term.c

@@ -58,6 +58,7 @@ static const struct tty_term_code_entry tty_term_codes[] = {
 	[TTYC_AX] = { TTYCODE_FLAG, "AX" },
 	[TTYC_BCE] = { TTYCODE_FLAG, "bce" },
 	[TTYC_BEL] = { TTYCODE_STRING, "bel" },
+	[TTYC_BIDI] = { TTYCODE_STRING, "Bidi" },
 	[TTYC_BLINK] = { TTYCODE_STRING, "blink" },
 	[TTYC_BOLD] = { TTYCODE_STRING, "bold" },
 	[TTYC_CIVIS] = { TTYCODE_STRING, "civis" },