@media screen {
    aside {
        position: fixed;
        left: 0;
        top: 53px;
        width: 300px;
        overflow-x: hidden;
        overflow-y: auto;
    }

    #main {
        top: 53px;
    }

    header, #main {
        position: absolute;
        left: 300px;
        right: 0;
    }
}