code: func (c *baseFlowController) getWindowUpdate() protocol.ByteCount {
  	if !c.hasWindowUpdate() {
  		return 0
  	}
  
  	c.maybeAdjustWindowSize()
  	c.receiveWindow = c.bytesRead + c.receiveWindowSize
  	return c.receiveWindow
  }
docs: // getWindowUpdate updates the receive window, if necessary
  // it returns the new offset