window onload does not fire up in chrome

[zilberlex]

Describe the bug

When i use <svelte:window onload={onLoad}> it does not work in chrome (And also not in svelte.dev environments), but it does work in Firefox.

After talking with chatgpt the fix was:

onMount(() => {
    if (document.readyState === 'complete') {
        onLoad();
    } else {
        // Otherwise, wait for the load event
        window.addEventListener('load', onLoad, { once: true });
    }
});

function onLoad(_event) {
    console.debug('loaded window.');
    isAppLoaded = true;
}

According to chatgpt onload event may fire before the sveltecode which attaches the eventhandler executes.

I am not a big expert in WebDev, but if I understand this correctly, it is not only a svelt issue. It would be nice if the had already the fix above implmeneted, if that is even the correct way to handle that.

Reproduction

https://svelte.dev/playground/0f493cd43f36479fb6c907fa08d16ed3?version=5.16.0

Currently, it is successfully reproduces in the online environemnt of svelte.dev, but due to the nature of the issue (race condition), there might be a chance it won't reproduce in the future.

Logs

No response

System Info

Independant Scripts:
  Chromium (131.0.2903.99)    

Internet Explorer: 11.0.26100.1882
  npmPackages:
    svelte: ^5.0.0 => 5.16.0

Severity

annoyance

[dummdidumm]

For <img onload> we did implement a solution so that it still fires even if the JS code didn't fully hydrate yet. For <svelte:window onload> we could implement another special-case solution (basically what chatgpt told you), or if we think that could teach wrong patterns instead straight away emit a warning when you use onload, stating that the event might never fire due to race conditions.

[teemingc]

similar to sveltejs/kit#11877

[trueadm]

I am not a big expert in WebDev, but if I understand this correctly, it is not only a svelt issue. It would be nice if the had already the fix above implmeneted, if that is even the correct way to handle that.

That means that onload will fire every time the component with <svelte:window onload> mounts though, which is clearly not right as the window only loads once. I actually think the correct approach is for you to do what you've done in your onMount function here and for Svelte to deprecate onload from <svelte:window> entirely – as mounting the app will always occur after the window event has been dispatched by the browser.