// refresh a part of the HTML specified by the given ID,
// by using the contents fetched from the given URL.
function refreshPart(id,url) {
    var f = function() {
        if(isPageVisible()) {
            new Ajax.Request(url, {
                onSuccess: function(rsp) {
                    var hist = $(id);
                    if (hist == null) {
                        console.log("There's no element that has ID of " + id);
                        return;
                    }
                    var p = hist.up();

                    var div = document.createElement('div');
                    div.innerHTML = rsp.responseText;

                    var node = $(div).firstDescendant();
                    p.replaceChild(node, hist);
 ########## hist cannot be garbage collected after the replaceChild!
 ########## If I comment out applySubtree(node) the memory leak disappears
 ########## So perhaps some event listeners stopping the GC from cleaning up?
                    Behaviour.applySubtree(node);
                    layoutUpdateCallback.call();

                    if(isRunAsTest) return;
                    refreshPart(id,url);
                }
            });    
        } else {
            // Reschedule
            if(isRunAsTest) return;
            refreshPart(id,url);
        }
        
    };
    // if run as test, just do it once and do it now to make sure it's working,
    // but don't repeat.
    if(isRunAsTest) f();
    else    window.setTimeout(f, 5000);
}