Add a format for number of bytes writtent to client, useful for debugging.

format.c

@@ -1154,6 +1154,7 @@ format_defaults_client(struct format_tree *ft, struct client *c)
 
 	format_add_tv(ft, "client_created", &c->creation_time);
 	format_add_tv(ft, "client_activity", &c->activity_time);
+	format_add(ft, "client_written", "%zu", tty->written);
 
 	name = server_client_get_key_table(c);
 	if (strcmp(c->keytable->name, name) == 0)

tmux.1

@@ -3521,6 +3521,7 @@ The following variables are available, where appropriate:
 .It Li "client_tty" Ta "" Ta "Pseudo terminal of client"
 .It Li "client_utf8" Ta "" Ta "1 if client supports utf8"
 .It Li "client_width" Ta "" Ta "Width of client"
+.It Li "client_written" Ta "" Ta "Bytes written to client"
 .It Li "command" Ta "" Ta "Name of command in use, if any"
 .It Li "command_list_name" Ta "" Ta "Command name if listing commands"
 .It Li "command_list_alias" Ta "" Ta "Command alias if listing commands"

tmux.h

@@ -1043,6 +1043,7 @@ struct tty {
 	struct evbuffer	*in;
 	struct event	 event_out;
 	struct evbuffer	*out;
+	size_t		 written;
 
 	struct termios	 tio;
 

tty.c

@@ -427,6 +427,7 @@ tty_add(struct tty *tty, const char *buf, size_t len)
 
 	evbuffer_add(tty->out, buf, len);
 	log_debug("%s: %.*s", c->name, (int)len, (const char *)buf);
+	tty->written += len;
 
 	if (tty_log_fd != -1)
 		write(tty_log_fd, buf, len);