Add support for the iTerm2 sychronized updates escape sequence which

drastically reduces flickering.

screen-redraw.c

@@ -435,6 +435,7 @@ screen_redraw_screen(struct client *c)
 
 	flags = screen_redraw_update(c, c->flags);
 	screen_redraw_set_context(c, &ctx);
+	tty_sync_start(&c->tty);
 
 	if (flags & (CLIENT_REDRAWWINDOW|CLIENT_REDRAWBORDERS)) {
 		if (ctx.pane_status != PANE_STATUS_OFF)
@@ -448,7 +449,9 @@ screen_redraw_screen(struct client *c)
 		screen_redraw_draw_status(&ctx);
 	if (c->overlay_draw != NULL && (flags & CLIENT_REDRAWOVERLAY))
 		c->overlay_draw(c, &ctx);
+
 	tty_reset(&c->tty);
+	tty_sync_end(&c->tty);
 }
 
 /* Redraw a single pane. */
@@ -461,9 +464,12 @@ screen_redraw_pane(struct client *c, struct window_pane *wp)
 		return;
 
 	screen_redraw_set_context(c, &ctx);
+	tty_sync_start(&c->tty);
 
 	screen_redraw_draw_pane(&ctx, wp);
+
 	tty_reset(&c->tty);
+	tty_sync_end(&c->tty);
 }
 
 /* Draw a border cell. */

screen-write.c

@@ -100,7 +100,8 @@ void
 screen_write_start(struct screen_write_ctx *ctx, struct window_pane *wp,
     struct screen *s)
 {
-	u_int	y;
+	struct tty_ctx	ttyctx;
+	u_int		y;
 
 	memset(ctx, 0, sizeof *ctx);
 
@@ -129,18 +130,26 @@ screen_write_start(struct screen_write_ctx *ctx, struct window_pane *wp,
 			    screen_size_y(ctx->s));
 		}
 	}
+
+	screen_write_initctx(ctx, &ttyctx);
+	tty_write(tty_cmd_syncstart, &ttyctx);
 }
 
 /* Finish writing. */
 void
 screen_write_stop(struct screen_write_ctx *ctx)
 {
+	struct tty_ctx	ttyctx;
+
 	screen_write_collect_end(ctx);
 	screen_write_collect_flush(ctx, 0);
 
 	log_debug("%s: %u cells (%u written, %u skipped)", __func__,
 	    ctx->cells, ctx->written, ctx->skipped);
 
+	screen_write_initctx(ctx, &ttyctx);
+	tty_write(tty_cmd_syncend, &ttyctx);
+
 	free(ctx->item);
 	free(ctx->list); /* flush will have emptied */
 }

tmux.1

@@ -5511,6 +5511,8 @@ $ printf '\e033[4 q'
 If
 .Em Se
 is not set, \&Ss with argument 0 will be used to reset the cursor style instead.
+.It Em \&Sync
+Show that the terminal supports synchronized updates.
 .It Em \&Tc
 Indicate that the terminal supports the
 .Ql direct colour

tmux.h

@@ -452,6 +452,7 @@ enum tty_code_code {
 	TTYC_SMUL,
 	TTYC_SMXX,
 	TTYC_SS,
+	TTYC_SYNC,
 	TTYC_TC,
 	TTYC_TSL,
 	TTYC_U8,
@@ -1181,6 +1182,7 @@ struct tty_term {
 #define TERM_DECSLRM 0x4
 #define TERM_DECFRA 0x8
 #define TERM_RGBCOLOURS 0x10
+#define TERM_SYNC 0x20
 	int		 flags;
 
 	LIST_ENTRY(tty_term) entry;
@@ -1949,6 +1951,8 @@ void	tty_set_title(struct tty *, const char *);
 void	tty_update_mode(struct tty *, int, struct screen *);
 void	tty_draw_line(struct tty *, struct window_pane *, struct screen *,
 	    u_int, u_int, u_int, u_int, u_int);
+void	tty_sync_start(struct tty *);
+void	tty_sync_end(struct tty *);
 int	tty_open(struct tty *, char **);
 void	tty_close(struct tty *);
 void	tty_free(struct tty *);
@@ -1976,6 +1980,8 @@ void	tty_cmd_scrolldown(struct tty *, const struct tty_ctx *);
 void	tty_cmd_reverseindex(struct tty *, const struct tty_ctx *);
 void	tty_cmd_setselection(struct tty *, const struct tty_ctx *);
 void	tty_cmd_rawstring(struct tty *, const struct tty_ctx *);
+void	tty_cmd_syncstart(struct tty *, const struct tty_ctx *);
+void	tty_cmd_syncend(struct tty *, const struct tty_ctx *);
 
 /* tty-term.c */
 extern struct tty_terms tty_terms;

tty-keys.c

@@ -1117,7 +1117,7 @@ tty_keys_device_status_report(struct tty *tty, const char *buf, size_t len,
 
 	/* Set terminal flags. */
 	if (strncmp(tmp, "ITERM2 ", 7) == 0)
-		flags |= (TERM_DECSLRM|TERM_256COLOURS|TERM_RGBCOLOURS);
+		flags |= (TERM_DECSLRM|TERM_256COLOURS|TERM_RGBCOLOURS|TERM_SYNC);
 	if (strncmp(tmp, "TMUX ", 5) == 0)
 		flags |= (TERM_256COLOURS|TERM_RGBCOLOURS);
 	log_debug("%s: received DSR %.*s", c->name, (int)*size, buf);

tty-term.c

@@ -260,6 +260,7 @@ static const struct tty_term_code_entry tty_term_codes[] = {
 	[TTYC_SMUL] = { TTYCODE_STRING, "smul" },
 	[TTYC_SMXX] =  { TTYCODE_STRING, "smxx" },
 	[TTYC_SS] = { TTYCODE_STRING, "Ss" },
+	[TTYC_SYNC] = { TTYCODE_FLAG, "Sync" },
 	[TTYC_TC] = { TTYCODE_FLAG, "Tc" },
 	[TTYC_TSL] = { TTYCODE_STRING, "tsl" },
 	[TTYC_U8] = { TTYCODE_NUMBER, "U8" },
@@ -532,6 +533,10 @@ tty_term_find(char *name, int fd, char **cause)
 	    tty_term_has(term, TTYC_SETRGBB)))
 		term->flags |= TERM_RGBCOLOURS;
 
+	/* Set flag if terminal has synchronized updates. */
+	if (tty_term_flag(term, TTYC_SYNC))
+		term->flags |= TERM_SYNC;
+
 	/*
 	 * Terminals without xenl (eat newline glitch) wrap at at $COLUMNS - 1
 	 * rather than $COLUMNS (the cursor can never be beyond $COLUMNS - 1).

tty.c

@@ -1426,6 +1426,20 @@ tty_draw_line(struct tty *tty, struct window_pane *wp, struct screen *s,
 	tty_update_mode(tty, tty->mode, s);
 }
 
+void
+tty_sync_start(struct tty *tty)
+{
+	if ((tty->term->flags|tty->term_flags) & TERM_SYNC)
+		tty_puts(tty, "\033P=1s\033\\");
+}
+
+void
+tty_sync_end(struct tty *tty)
+{
+	if ((tty->term->flags|tty->term_flags) & TERM_SYNC)
+		tty_puts(tty, "\033P=2s\033\\");
+}
+
 static int
 tty_client_ready(struct client *c, struct window_pane *wp)
 {
@@ -1919,6 +1933,18 @@ tty_cmd_rawstring(struct tty *tty, const struct tty_ctx *ctx)
 	tty_invalidate(tty);
 }
 
+void
+tty_cmd_syncstart(struct tty *tty, __unused const struct tty_ctx *ctx)
+{
+	tty_sync_start(tty);
+}
+
+void
+tty_cmd_syncend(struct tty *tty, __unused const struct tty_ctx *ctx)
+{
+	tty_sync_end(tty);
+}
+
 static void
 tty_cell(struct tty *tty, const struct grid_cell *gc, struct window_pane *wp)
 {