Fails to guard against zero-length array

Apologies for no jFiddle etc. here, this was in a program of 100,000+ lines of JS; I know exactly the fix needed, it's a simple piece of defensive programming.

In _createOverlay: function(), you have code as follows:

    if ( !this._allowInteraction( event ) ) {
        event.preventDefault();
        this._trackingInstances()[ 0 ]._focusTabbable();
    }

There are definitely circumstances where this._trackingInstances() can be an empty array, so this should be something like:

    if ( !this._allowInteraction( event ) ) {
        event.preventDefault();
        if (this._trackingInstances().length) { 
            this._trackingInstances()[ 0 ]._focusTabbable();
        }
    }

It might be better to put the return of this._trackingInstances() in a local variable to avoid calling it twice; I leave that to you. But it is definitely possible for the array returned to be an empty array, in which case this._trackingInstances()[ 0 ]._focusTabbable() will certainly throw an error in the existing code.