QWebSocket "Invalid UTF-8 Code Encountered" Error in Qt6

Description: In a scenario where two Qt programs communicate via QWebSocket, the receiving side occasionally closes the socket with the error "Invalid UTF-8 code encountered." The sending program always uses the following code to transmit UTF-8 encoded JSON documents:
QJsonDocument doc(aQJsonObject);
client->sendTextMessage(doc.toJson());

This issue occurs after migrating from Qt5 to Qt6, where QTextCodec was replaced by QStringDecoder, which defaults to QStringDecoder::Flag::Stateless. It has been observed that removing the Stateless flag resolves the issue, suggesting that stateless decoding may be the cause.

Environment:

• OS: OpenSUSE Tumbleweed

• Compiler: GCC 14.2

• Qt Version: Qt6

Suspected Cause: The change in decoding behavior between Qt5 and Qt6 (from QTextCodec to QStringDecoder with the Stateless flag) may be the cause of the error.