MooTools 1.6 Adaptions
Using MooTools 1.6 alongside CKEditor 5 caused problems.
I tracked the issue down to the browser module of MooTools 1.6 core.
Removing the following lines resolves the issue:
this.Document = document.$constructor = new Type('Document', function(){});
Document.mirror(function(name, method){
document[name] = method;
});
These lines could/should be removed from our MooTools build.