I figured out this happens with Safari's old versions rather than 15.3

mdn/browser-compat-data#15592

Adding a polyfill should work:

if (!Array.prototype.at) {
  Array.prototype.at = function at(n) {
    let i = Math.trunc(n) || 0

    if (i < 0) i += this.length

    if (i < 0 || i >= this.length) return undefined

    return this[i]
  }
}

Took from https://github.com/vercel/next.js/pull/44436/files#diff-2f5596fa58e0c6b1fc2c6453e3a99ba4e25c585b657625c58f019b9e044e766cR133-R143

@ericpaulsen this is related to old browser versions, so I would like to know from you how frequently we see customers using browser versions +2 years old.

@BrunoQuaresma i don't have any data on our customer browser versions. although I am consistently seeing this behavior on Firefox 119 (latest).

It was not related to the .at this time that I tried it 🤔. It is a known "bug" when dealing with incoming messages when the connection is closed socketio/socket.io#2016.

In our case, this happens because the "agent logs" can be toggled, and depending on whether that is open or not, it will close or open the connection. One way to reproduce this is to open the agent logs section and close it immediately.