Add an option (default off) to control the passthrough escape sequence.

Like set-clipboard and allow-rename it is safer to forbid this by
default.

input.c

@@ -2235,15 +2235,19 @@ input_enter_dcs(struct input_ctx *ictx)
 static int
 input_dcs_dispatch(struct input_ctx *ictx)
 {
+	struct window_pane	*wp = ictx->wp;
 	struct screen_write_ctx	*sctx = &ictx->ctx;
 	u_char			*buf = ictx->input_buf;
 	size_t			 len = ictx->input_len;
 	const char		 prefix[] = "tmux;";
 	const u_int		 prefixlen = (sizeof prefix) - 1;
 
+	if (wp == NULL)
+		return (0);
 	if (ictx->flags & INPUT_DISCARD)
 		return (0);
-
+	if (!options_get_number(ictx->wp->options, "allow-passthrough"))
+		return (0);
 	log_debug("%s: \"%s\"", __func__, buf);
 
 	if (len >= prefixlen && strncmp(buf, prefix, prefixlen) == 0)

options-table.c

@@ -302,7 +302,7 @@ const struct options_table_entry options_table[] = {
 	  .choices = options_table_extended_keys_list,
 	  .default_num = 0,
 	  .text = "Whether to request extended key sequences from terminals "
-	          "that support it."
+		  "that support it."
 	},
 
 	{ .name = "focus-events",
@@ -801,6 +801,14 @@ const struct options_table_entry options_table[] = {
 		  "linked to ('off')."
 	},
 
+	{ .name = "allow-passthrough",
+	  .type = OPTIONS_TABLE_FLAG,
+	  .scope = OPTIONS_TABLE_WINDOW|OPTIONS_TABLE_PANE,
+	  .default_num = 0,
+	  .text = "Whether applications are allowed to use the escape sequence "
+	          "to bypass tmux."
+	},
+
 	{ .name = "allow-rename",
 	  .type = OPTIONS_TABLE_FLAG,
 	  .scope = OPTIONS_TABLE_WINDOW|OPTIONS_TABLE_PANE,
@@ -989,7 +997,7 @@ const struct options_table_entry options_table[] = {
 	  .choices = options_table_pane_border_lines_list,
 	  .default_num = PANE_LINES_SINGLE,
 	  .text = "Type of characters used to draw pane border lines. Some of "
-	          "these are only supported on terminals with UTF-8 support."
+		  "these are only supported on terminals with UTF-8 support."
 	},
 
 	{ .name = "pane-border-status",
@@ -1041,7 +1049,7 @@ const struct options_table_entry options_table[] = {
 	  .choices = options_table_popup_border_lines_list,
 	  .default_num = BOX_LINES_SINGLE,
 	  .text = "Type of characters used to draw popup border lines. Some of "
-	          "these are only supported on terminals with UTF-8 support."
+		  "these are only supported on terminals with UTF-8 support."
 	},
 
 	{ .name = "remain-on-exit",
@@ -1171,7 +1179,7 @@ const struct options_table_entry options_table[] = {
 	  .scope = OPTIONS_TABLE_WINDOW,
 	  .default_num = 1,
 	  .text = "Whether xterm-style function key sequences should be sent. "
-	          "This option is no longer used."
+		  "This option is no longer used."
 	},
 
 	/* Hook options. */
@@ -1219,8 +1227,8 @@ const struct options_table_entry options_table[] = {
 	OPTIONS_TABLE_HOOK("client-active", ""),
 	OPTIONS_TABLE_HOOK("client-attached", ""),
 	OPTIONS_TABLE_HOOK("client-detached", ""),
- 	OPTIONS_TABLE_HOOK("client-focus-in", ""),
+	OPTIONS_TABLE_HOOK("client-focus-in", ""),
- 	OPTIONS_TABLE_HOOK("client-focus-out", ""),
+	OPTIONS_TABLE_HOOK("client-focus-out", ""),
 	OPTIONS_TABLE_HOOK("client-resized", ""),
 	OPTIONS_TABLE_HOOK("client-session-changed", ""),
 	OPTIONS_TABLE_PANE_HOOK("pane-died", ""),

tmux.1

@@ -4403,6 +4403,13 @@ The default is on.
 Available pane options are:
 .Pp
 .Bl -tag -width Ds -compact
+.It Xo Ic allow-passthrough
+.Op Ic on | off
+.Xc
+Allow programs in the pane to bypass
+.Nm
+using a terminal escape sequence (\eePtmux;...\ee\e\e).
+.Pp
 .It Xo Ic allow-rename
 .Op Ic on | off
 .Xc