SSE reconnect never gives up and retries without jitter

[bjinwright]

Summary

htmx-ext-sse@2.2.4 (and src/sse/sse.js on main) still reconnects forever with no jitter after EventSource.CLOSED. The old XOR backoff (2 ^ retryCount) was fixed in 2.2.1; these two problems replaced it.

Happy to open a PR if you want one. CONTRIBUTING prefers an issue first for discussion, so this stops here until you say otherwise.

Current code

ensureEventSource's source.onerror in src/sse/sse.js (same in the 2.2.4 npm file):

if (source.readyState === EventSource.CLOSED) {
  retryCount = retryCount || 0
  retryCount = Math.max(Math.min(retryCount * 2, 128), 1)
  var timeout = retryCount * 500
  window.setTimeout(function() {
    ensureEventSourceOnElement(elt, retryCount)
  }, timeout)
}

1. No give-up

Math.min(retryCount * 2, 128) caps the delay at 64 seconds (128 * 500). There is no maximum-attempts check. A permanently unreachable endpoint is retried forever at 64s intervals for as long as the element stays in the DOM.

onopen resets retryCount to 0 after a successful reconnect, which is good for a transient outage. It does not bound a never-recovering endpoint.

2. No jitter

Math.random() went away with the XOR expression in 2.2.1. Every client that lost the connection at the same moment now retries in lockstep: 0.5s, 1s, 2s, 4s, … A server that just came back gets a thundering herd.

Not this issue

2.0.0–2.2.0 used Math.random() * (2 ^ retryCount) * 500 plus Math.min(7, retryCount + 1). That XOR / capped-counter bug is already gone. Please do not treat this as a request to restore that expression.

Suggested shape (not a patch)

• Stop after a finite number of failed attempts (or expose a max-attempts option; Infinity can stay the default if you want today's behavior).
• Add jitter around the exponential delay so clients desynchronize.
• Keep the existing successful-open reset so a recovered stream does not stay at the 64s cap.

Verified against npm htmx-ext-sse@2.2.4 (2025-10-18) and bigskysoftware/htmx-extensions main src/sse/sse.js.