Add the following style sheet to the SMPortalPage.master to solve this issue:

 

<style>
body.v4master {
                height:inherit;
                width:inherit;
                overflow:visible!important; //!important to override the original theme.
}

body #s4-workspace {
                overflow-y:auto !important;
                overflow-x:auto !important;
                height:auto !important;
}
</style>

 

Source: http://blogs.technet.com/b/servicemanager/archive/2012/07/05/fixing-the-missing-scroll-bar-issue-on-the-scsm-2012-portal.aspx