private double getNodeThickness(final TreeNode treeNode) {
        return getWidthOrHeightOfNode(treeNode, !isLevelChangeInYAxis());
    }
When the level changes in Y-axis (i.e. root location Top or Bottom) the
height of a node is its thickness, otherwise the node's width is its
thickness.
<p>
The thickness of a node is used when calculating the locations of the
levels.

@param treeNode
@return
stroom-core-client-widget/src/main/java/stroom/widget/htree/client/treelayout/AbegoTreeLayout.java