code: func (c *baseFlowController) IsNewlyBlocked() (bool, protocol.ByteCount) {
  	if c.sendWindowSize() != 0 || c.sendWindow == c.lastBlockedAt {
  		return false, 0
  	}
  	c.lastBlockedAt = c.sendWindow
  	return true, c.sendWindow
  }
docs: // IsNewlyBlocked says if it is newly blocked by flow control.
  // For every offset, it only returns true once.
  // If it is blocked, the offset is returned.