How can I ignore window.onpopstate on page load?

Check for boolean truth of event.state in popstate event handler:

window.addEventListener('popstate', function(event) {
    if (event.state) {
        alert('!');
    }
}, false);

To ensure this will work, always specify a non-null state argument when calling history.pushState() or history.replaceState(). Also, consider using a wrapper library like History.js that provides consistent behavior across browsers.