There seems to be a little bug in the TreeView-Script.
If you have a parent with exactly one child it doesen't appear as folder (it still has the file-icon but is expandable).

The mistake is in the TreeView.js (and of course in the full-library-version, as well as in the minified one).

The following Line needs to be changed from:
Code:
            tNode.__eElement.cells[1].className = (tNode.__aSubItems.length > 1 || tNode.bHasChildren ? "treeFolder " : "treeIcon ") + (tNode.sIconClass || "");
to
Code:
            tNode.__eElement.cells[1].className = (tNode.__aSubItems.length > 0 || tNode.bHasChildren ? "treeFolder " : "treeIcon ") + (tNode.sIconClass || "");
Hint: search for "treeFolder" (should only appear on this line)

[edit] I'm using Ajax-library 2.4