Restore window.scrollTo in iOS Safari after tapping the status bar

jquery.restoreIOSscrollTo.js

/** 
  **UPDATE**: This was fixed in iOS 8. But this could still be valid for iOS 7 and below. 
  
  I believe this was first noticed here: http://blog.b123400.net/window-scrollto-and-ios-status-bar/
  
  > When the status bar is tapped, the page scrolls to the top and url bar is shown, 
  > then window.scrollTo doesn't work anymore.
  
  Yep!
  
  In the same post, Mr. b123400 notes:
  
  > To re-enable it, either:
  > 
  > 1. User scrolls the page, or
  > 2. Trigger window.scrollTo by user interaction, such as click and touchend

  This is my attempt to do #2 automatically, which should 'fix' window.scrollTo elsewhere 
  
  Started talking about it here: 
  https://github.com/julianshapiro/velocity/issues/282

  TODO: 
   - remove the jQuery dependency? nothing too tricky here.
   - better user agent sniff? I already don't do this for iOS Chrome... how about Opera?
*/

;(function ($, window, document, undefined) {
'use strict';

// Bail if not iOS or is iOS Chrome 
// https://developer.chrome.com/multidevice/user-agent
if ( ! /iPhone|iPad|iPod/.test(navigator.userAgent) || navigator.userAgent.indexOf('CriOS') > -1 ) { return; }

// thx: http://davidwalsh.name/javascript-debounce-function
var debounce = function(func, wait) {
  var timeout;
  return function() {
    var context = this, args = arguments;
    clearTimeout(timeout);
    timeout = setTimeout(function() {
      timeout = null;
      func.apply(context, args);
    }, wait);
  };
};

var checkAndAddTouchEvent = debounce( function() {

  if ( $(window).scrollTop() !== 0 ) { return; }
  
  $(document).one('touchstart', function() {
    /*
      This is the magic bit: calling window.scrollTo in a touchend/touchstart/click handler
      seems to break iOS Safari out of its "na-na-not-listening!" funk
    
      touchstart seems like the way to go
    
      That's my big assumption here: someone will probably touch *something* 
      before you have to fire window.scrollTo

      we're already at the top, so in theory scrollTo(0,0) this should have no effect
      other than allowing window.scrollTo to work elsewhere
    
      UPDATE: window.scrollBy also works, so that's even less likely to cause trouble
    */
    window.scrollBy(0,0);
  });

}, 500 ); // 500ms seems like enough time

$(window).on('scroll', checkAndAddTouchEvent);

}(jQuery, this, this.document));