Sorry, what I stated above is associated with another bug:
When condition meets: (hideRootNode && currentNode.getNextSibling() == null
&& currentNode == rootTreeNode.getFirstChild())
it should have two situation expanded=true and expanded=false, but source code only have one situation.

Sorry, I disagree with <the setExpanded attribute should be set to "true" when the hideRootNode="true">. When hideRootNode=true, the root's first child may have its children. So, the first node may expanded=true or expanded=false.

(root 0) //hidden
[+]node 1 //expanded=false

(root 0) //hidden
[-]node 1 //expanded=true

This is not a bug, as Brian Truitt mentioned that the setExpanded attribute should be set to "true" when the hideRootNode="true".

It seems that my code above fix this bug as follows: when hideRootNode = true, rootNode only have one child (currentNode.getNextSibling() == null && currentNode == rootTreeNode.getFirstChild()), then expanded = true or expanded=false always display "". It should be "" when expanded=true, and "-" when expanded=false.

I think there is a bug in TreeRenderer encodeNode method (line 500). Following is my suggestion:

This is current code:
if (currentNode.isRoot() && !isExpanded ||
hideRootNode
&& currentNode.getNextSibling() == null
&& currentNode == treeComponentRootNode.getFirstChild()
)

It should be:
if (currentNode.isRoot() || hideRootNode
&& currentNode.getNextSibling() == null
&& currentNode == rootTreeNode.getFirstChild()) {
if (isExpanded)

else

}

When you set hideRootNode to "true" did you remember to take your root node and setExpanded to true?

If the root node is both hidden and not expanded then you won't get a tree rendered...