put back r1.68 which allows an empty group to be sticky; behavior

change noticed by Thomas Pfaff and diagnosis why we need to
group_setactive in this case by Alexander Polakov.  replace XXX with
a useful comment.

group.c

@@ -269,8 +269,12 @@ group_hidetoggle(struct screen_ctx *sc, int idx)
 
 	if (gc->hidden)
 		group_show(sc, gc);
-	else
+	else {
 		group_hide(sc, gc);
+		/* make clients stick to empty group */
+		if (TAILQ_EMPTY(&gc->clients))
+			group_setactive(sc, idx);
+	}
 }
 
 void