Add a format to show if there are unseen changes while in a mode, from

Dan Aloni in GitHub issue 3498.

format.c

@@ -1885,6 +1885,18 @@ format_cb_pane_input_off(struct format_tree *ft)
 	return (NULL);
 }
 
+/* Callback for pane_unseen_changes. */
+static void *
+format_cb_pane_unseen_changes(struct format_tree *ft)
+{
+	if (ft->wp != NULL) {
+		if (ft->wp->flags & PANE_UNSEENCHANGES)
+			return (xstrdup("1"));
+		return (xstrdup("0"));
+	}
+	return (NULL);
+}
+
 /* Callback for pane_last. */
 static void *
 format_cb_pane_last(struct format_tree *ft)
@@ -2953,6 +2965,9 @@ static const struct format_table_entry format_table[] = {
 	{ "pane_tty", FORMAT_TABLE_STRING,
 	  format_cb_pane_tty
 	},
+	{ "pane_unseen_changes", FORMAT_TABLE_STRING,
+	  format_cb_pane_unseen_changes
+	},
 	{ "pane_width", FORMAT_TABLE_STRING,
 	  format_cb_pane_width
 	},

input.c

@@ -971,6 +971,10 @@ input_parse_buffer(struct window_pane *wp, u_char *buf, size_t len)
 	window_update_activity(wp->window);
 	wp->flags |= PANE_CHANGED;
 
+	/* Flag new input while in a mode. */
+	if (!TAILQ_EMPTY(&wp->modes))
+		wp->flags |= PANE_UNSEENCHANGES;
+
 	/* NULL wp if there is a mode set as don't want to update the tty. */
 	if (TAILQ_EMPTY(&wp->modes))
 		screen_write_start_pane(sctx, wp, &wp->base);

tmux.1

@@ -5257,6 +5257,7 @@ The following variables are available, where appropriate:
 .It Li "pane_title" Ta "#T" Ta "Title of pane (can be set by application)"
 .It Li "pane_top" Ta "" Ta "Top of pane"
 .It Li "pane_tty" Ta "" Ta "Pseudo terminal of pane"
+.It Li "pane_unseen_changes" Ta "" Ta "1 if there were changes in pane while in mode"
 .It Li "pane_width" Ta "" Ta "Width of pane"
 .It Li "pid" Ta "" Ta "Server PID"
 .It Li "rectangle_toggle" Ta "" Ta "1 if rectangle selection is activated"

tmux.h

@@ -1045,6 +1045,7 @@ struct window_pane {
 #define PANE_STATUSDRAWN 0x400
 #define PANE_EMPTY 0x800
 #define PANE_STYLECHANGED 0x1000
+#define PANE_UNSEENCHANGES 0x2000
 
 	int		 argc;
 	char	       **argv;

window.c

@@ -1120,6 +1120,7 @@ window_pane_reset_mode(struct window_pane *wp)
 
 	next = TAILQ_FIRST(&wp->modes);
 	if (next == NULL) {
+		wp->flags &= ~PANE_UNSEENCHANGES;
 		log_debug("%s: no next mode", __func__);
 		wp->screen = &wp->base;
 	} else {