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

Dan Aloni in GitHub issue 3498.

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 {