onEvent := func(e QUICEvent, src, dst *testQUICConn) bool {
		if e.Kind == QUICWriteData {
			// Provide the data one byte at a time.
			for i := range e.Data {
				if err := dst.conn.HandleData(e.Level, e.Data[i:i+1]); err != nil {
					t.Errorf("HandleData: %v", err)
					break
				}
			}
			return true
		}
		return false
	}
	if err := runTestQUICConnection(context.Background(), cli, srv, onEvent); err != nil {

	for range q.conn.quic.blockedc {
		// Wait for the handshake goroutine to return.
	}
	return q.conn.handshakeErr
}

// HandleData handles handshake bytes received from the peer.
// It may produce connection events, which may be read with [QUICConn.NextEvent].
func (q *QUICConn) HandleData(level QUICEncryptionLevel, data []byte) error {
	c := q.conn
	if c.in.level != level {