The UNDERTOW-2534 fix sets the TCCL to the MessageHandler class before invoking it. That works ok if the MessageHandler impl is provided by the deployment, but in the case of annotated endpoint methods (e.g. @OnMessage) the MessageHandler impl is an Undertow class, so TCCL is set to one that doesn't expose deployment classes.

This breaks applications that expect the TCCL to expose the app classes. One such application is in a websocket test in the EE 10 Platform TCK: https://github.com/jakartaee/platform-tck/blob/10.0.x/src/com/sun/ts/tests/websocket/ee/jakarta/websocket/containerprovider/metainf/WSClient.java#L39-L52