...
	// Adds or removes a child to the view by online invaliding
	// the minimal required portions of the cache, namely, the
	// old and new parent and the child.
	else if (change instanceof mxChildChange)
	{
		var newParent = this.model.getParent(change.child);
		this.view.invalidate(change.child, true, true);
		
		if (!this.model.contains(newParent) || this.isCellCollapsed(newParent))
		{
			this.view.invalidate(change.child, true, true);
			this.removeStateForCell(change.child);
			
			// Handles special case of current root of view being removed
			if (this.view.currentRoot == change.child)
			{
				this.home();
			}
		}
...

/**
 * Function: isAncestor
 * 
 * Returns true if the given parent is an ancestor of the given child. Note 
 * returns true if child == parent.
 *
 * Parameters:
 * 
 * parent - <mxCell> that specifies the parent.
 * child - <mxCell> that specifies the child.
 */
mxGraphModel.prototype.isAncestor = function(parent, child)
{
	while (child != null && child != parent)
	{
		child = this.getParent(child);
	}
	
	return child == parent;
};