carrotquest.addCallback('messenger_opened', function() {
    var css = '#cbh{display: none;}',
        head = document.head || document.getElementsByTagName('head')[0],
        style = document.createElement('style');
    style.type = 'text/css';
    if (style.styleSheet){
        style.styleSheet.cssText = css;
    } else {
        style.appendChild(document.createTextNode(css));
    };
    head.appendChild(style);
});

carrotquest.addCallback('messenger_closed', function() {
    var css = '#cbh{display: block;}',
        head = document.head || document.getElementsByTagName('head')[0],
        style = document.createElement('style');
    style.type = 'text/css';
    if (style.styleSheet){
        style.styleSheet.cssText = css;
    } else {
        style.appendChild(document.createTextNode(css));
    };
    head.appendChild(style);
});