leveled term view: only notify when level is changed

nested/src/leveled_term_view.rs

@@ -57,6 +57,7 @@ impl LeveledTermView {
     }
 
     pub fn set_level(&mut self, l: usize) {
+        if self.level != l {
             self.level = l;
 
             // update complete area
@@ -64,6 +65,7 @@ impl LeveledTermView {
                 self.cast.notify_each(a);
             }
         }
+    }
 }
 
 impl ImplIndexView for LeveledTermView {