code: func (w *window) waitWriterBlocked() {
  	w.Cond.L.Lock()
  	for w.writeWaiters == 0 {
  		w.Cond.Wait()
  	}
  	w.Cond.L.Unlock()
  }
docs: // waitWriterBlocked waits until some goroutine is blocked for further
  // writes. It is used in tests only.