I had this problem too and put this in the custom css. It's kind of hacky but the users in my forum like it better this way:

@media (max-height: 500px) { 
    .composer {
        position: static;
        min-height:9em;
        padding-top:1em;
    }
    .composer .write-preview-container {
        min-height: 5em;
    }
    body {
        margin-bottom:0!important;
    }
}