Strange behavior with the ACE Editor's UndoManager

Question

Using the ACE Editor, it appears as if using editor.getSession().getUndoManager() with a session that has been set with editor.setSession() returns an UndoManager object that contains only empty functions for undo(), redo(), and reset(). It does not return an UndoManager with methods like hasUndo(), or hasRedo(), or any of the other documented methods. In the source, it looks like my the session must not have an $undoManager and is forced to use an empty (and incomplete) $defaultUndoManager:

this.$defaultUndoManager = {
    undo: function() {},
    redo: function() {},
    reset: function() {}
};

this.getUndoManager = function() {
    return this.$undoManager || this.$defaultUndoManager;
};

Why would a session that was set with editor.setSession() have an $undoManager that is undefined like that?

Answer 1

Looks like one is supposed to set undo manager on the session like this (ace.js#L112)
And if undomamager is missing defaultUndomanager is returned so that editor methods do not have to check for null, when accessing undomanager.
I think the reason EditSession doesn't create undoManager by itself is to allow users to provide special implementations of undomanager (e.g. for collaboration)