I consider it more a documentation typo than a bug…
From the class reference (CTreeView url property):
Quote
When the page is displayed, the browser will request this URL with a GET parameter named ‘source’ whose value is ‘root’. The server script should then generate the needed tree view data corresponding to the root of the tree (see saveDataAsJson.) When a node has a CSS class ‘hasChildren’, then expanding this node will also cause a dynamic loading of its child nodes. In this case, the value of the ‘source’ GET parameter is the ‘id’ property of the node.
It took me some time to realize that the parameter that is passed is called 'root' instead of 'source'. Its value uppon first request is 'source', when you expand nodes, its value is the id property of the node that is to be expanded.