In the current WebsocketChannel API it is possible to set an idle timeout through: channel.setIdleTimeout().
It seems like the resulting CloseMessage out of a timeout is:

code = 1001 (Going Away)
reason = ""

So there is no way for a client to differentiate between a timeout from a normal server close.

I am not using the JSR extension but I know that in the JSR spec, they use 1006.

Would it be possible to either set a specific CloseMessage code, or a specific CloseMessage reason, or the ability for the application to catch the timeout close event and send a custom CloseMessage.

I am also interested to be able to catch the timeout event for metric purposes. Maybe it is possible with the current API and I missed the way to do it.

I can do a PR given a little bit of guidance.