private TreeNode nextRight(final TreeNode v) {
        return tree.isLeaf(v)
                ? getThread(v)
                : tree.getLastChild(v);
    }
The distance of two nodes is the distance of the centers of both noded.
<p>
I.e. the distance includes the gap between the nodes and half of the
sizes of the nodes.

@param v
@param w
@return the distance between node v and w
stroom-core-client-widget/src/main/java/stroom/widget/htree/client/treelayout/AbegoTreeLayout.java