Facebook Callback appends ‘#_=_’ to Return URL

via Facebook’s Platform Updates:

Change in Session Redirect Behavior

This week, we started adding a fragment #____=____ to the redirect_uri when
this field is left blank. Please ensure that your app can handle this
behavior.

To prevent this, set the redirect_uri in your login url request like so: (using Facebook php-sdk)

$facebook->getLoginUrl(array('redirect_uri' => $_SERVER['SCRIPT_URI'],'scope' => 'user_about_me'));

UPDATE

The above is exactly as the documentation says to fix this. However, Facebook’s documented solution does not work. Please consider leaving a comment on the Facebook Platform Updates blog post and follow this bug to get a better answer. Until then, add the following to your head tag to resolve this issue:

<script type="text/javascript">
    if (window.location.hash && window.location.hash == '#_=_') {
        window.location.hash="";
    }
</script>

Or a more detailed alternative (thanks niftylettuce):

<script type="text/javascript">
    if (window.location.hash && window.location.hash == '#_=_') {
        if (window.history && history.pushState) {
            window.history.pushState("", document.title, window.location.pathname);
        } else {
            // Prevent scrolling by storing the page's current scroll offset
            var scroll = {
                top: document.body.scrollTop,
                left: document.body.scrollLeft
            };
            window.location.hash="";
            // Restore the scroll offset, should be flicker free
            document.body.scrollTop = scroll.top;
            document.body.scrollLeft = scroll.left;
        }
    }
</script>