first, thanks for HTMLUnit, first time I am using it and it is really nice.

I tried to test a javalin web app with SSE (server side events), plus htmx on the client side (you tweeted about it a few weeks ago, see https://twitter.com/HtmlUnit/status/1447250140423770122).

So, here is my setup:

    private final Javalin app;
    private final ConcurrentLinkedQueue<SseClient> clients = new ConcurrentLinkedQueue<>();
    app.sse("sse-events", client -> {
        clients.add(client);
        client.onClose(() -> clients.remove(client));
    });

<div class="container mx-auto px-40 pb-10">
    <div hx-sse="connect:/sse-events">
        <div hx-sse="swap:box-update">
            <div class="flex pl-4 pr-4 pt-4 pb-4 mt-4 mb-4" id="sse-element">
                <div class="flex-grow content text-text">&nbsp;</div>
            </div>
        </div>
    </div>
</div>

Test looks like this

try (final WebClient webClient = new WebClient()) {
    webClient.getOptions().setRedirectEnabled(true);
    webClient.getOptions().setCssEnabled(false);
    webClient.setAjaxController(new NicelyResynchronizingAjaxController());
    webClient.setJavaScriptErrorListener(new DefaultJavaScriptErrorListener());
    final HtmlPage mainPage = webClient.getPage("http://localhost:7000/");
    ...
}

Running that test in debug mode, leads to the sse-events endpoint not being called, despite part of the HTML that the mainpage returns.

Hope this helps as a start, otherwise I can try to provide a minimal fully reproducible example.