fullscreen.css

:-webkit-full-screen .gist {
  overflow-y:auto;
}

include.js

// include.js
// ----------
// Code for included the full screen code snippets on your site

var s = document.createElement('script');
s.src = 'https://raw.github.com/gist/3784818/c69646def2328f589aeb632615cdca099ec80c62/snippets.js';
document.getElementsByTagName('head')[0].appendChild(s);

snippets.js

// full screen code snippets

(function() {
  "use strict";

  function addCss() {
    var css = 'html:-webkit-full-screen .gist, html:fullscreen { height: 100%; overflow:auto; width: 100%; }';
    var styleTag = document.createElement('style');
    styleTag.innerHTML = css;
    document.getElementsByTagName('head')[0].appendChild(styleTag);
  }

  function fullScreenerise(gist) {
   // addCss();
    gist.addEventListener('click', function() {
      if (gist.requestFullScreen) {
        gist.requestFullScreen();
      } else if (gist.mozRequestFullScreen) {
        gist.mozRequestFullScreen();
      } else if (gist.webkitRequestFullScreen) {
        gist.webkitRequestFullScreen();
      }
    }, true);
  }

  if (document.querySelectorAll) {
    var gists = document.querySelectorAll('.gist');
    for (var i = 0, length = gists.length; i < length; i++) {
      fullScreenerise(gists[i]);
    }
  }
})();