MooTools 1.1 is no longer supported by the developers, but it is still used by Joomla!

I.e. with window.scrollTo() or Fx.Scroll.toElement() methods.

We have discovered that the problem is just 1 string of code in window.getHeight() method:

getHeight: function(){
if (this.webkit419) return this.innerHeight;
if (this.opera) return document.body.clientHeight;
return document.documentElement.clientHeight;
}

The special Opera case is useless here and produce errors, so we can repair it by adding our own function:

window.addEvent('domready', function() {
window.extend({
getHeight: function(){
if (this.webkit419) return this.innerHeight;
return document.documentElement.clientHeight;
}
});
});

This helps to use our own files and not to modify core MooTools file.