Does htmx's SSE extension guarantee reconnection after browser tab restoration across all supported browsers?

Answer

No, the htmx SSE extension does not guarantee reconnection after a browser tab is restored (whether from background or from the back‑forward cache) across all supported browsers.

Why the guarantee does not exist

The extension relies on the native EventSource API. It only exposes a reconnect option that sets a custom retry interval passed to EventSource, but it does not intercept pagehide or pageshow events to restart the connection manually. Consequently, resumption depends entirely on how each browser’s EventSource implementation handles network loss and tab restoration.

What the reconnect parameter does

Setting reconnect: 2000 (or any millisecond value) tells htmx to pass that value as the optional retry field when creating the EventSource. If the server does not send its own retry directive, EventSource will use that number as its base interval, but the actual back‑off algorithm (exponential in Chrome/Firefox, different in Safari) remains browser‑specific.

Practical verification steps

1. Create a minimal page that includes htmx and the SSE extension, sets reconnect to a short value (e.g., 2000), and opens an SSE endpoint.
2. Open the page, let the connection establish, then background the tab (or lock the screen) for several seconds.
3. Restore the tab and observe whether the SSE stream resumes without a page reload.
4. Repeat the test in Chrome, Firefox, and Safari (including versions that support bfcache) and note any differences in reconnection latency or failure.