Wikimedia Bugzilla is closed!

I couldn't find a way to quickly add this myself, but this needs a title="" element:

        // Add new messages link.
        $url = SpecialPage::getTitleFor( 'NewMessages' )->getLocalURL();
        $msg = $newMessagesCount ? 'lqt-newmessages-n' : 'lqt_newmessages';
        $newMessagesLink = array(
            'href' => $url,
            'text' => wfMsg( $msg, $wgLang->formatNum( $newMessagesCount ) ),
            'active' => $newMessagesCount > 0,
        );

Then you'd get a text when you hover over it like the rest of the toolbar buttons.