fn.js

function resizeNav() {
    // Set the nav height to fill the window
    $("#nav-fullscreen").css({
        "height": window.innerHeight
    });

    // Set the circle radius to the length of the window diagonal,
    // this way we're only making the circle as big as it needs to be.
    var radius = Math.sqrt(Math.pow(window.innerHeight, 2) + Math.pow(window.innerWidth, 2));
    var diameter = radius * 2;
    $("#nav-overlay").width(diameter);
    $("#nav-overlay").height(diameter);
    $("#nav-overlay").css({
        "margin-top": -radius,
        "margin-left": -radius
    });
}

// Set up click and window resize callbacks, then init the nav.
$(document).ready(function () {
    $("#nav-toggle").click(function () {
        $("#nav-toggle, #nav-overlay, #nav-fullscreen").toggleClass("open");
    });

    $(window).resize(resizeNav);

    resizeNav();
});