What is the default buffer size of websocket.Conn.Send?

[GingerMoon]

I am using gorilla websocket as a client. the server is python fastapi websocket.
I am keeping sending data to the server via websocket.Conn.Send.
But I find that the python fastapi server close the connection after a while.
It seems like the server hasn't received data for a while, although I keep sending data to the server.
But after I change to use NextWriter , the server doesn't close the connection.
Is NextWriter for the purpose of flush?
Why the server doesn't receive my data when I use websocket.Conn.Send?
What is the default buffer size?
Any insight would be much apprecaited!