My workaround is to navigate through elements of treeview dom to get "show more" element with
public static List<Element> findElements(Element element) {
    ArrayList<Element> result = new ArrayList<Element>();
    findShowMore(result, element);  return result; }
private static void findShowMore(ArrayList res, Element element) {
    String c;
    if (element == null) {      return;     }
    if (element.getInnerText().equals("Show more")) {       res.add(element);
    }
    for (int i = 0; i < DOM.getChildCount(element); i++) {      Element
child = DOM.getChild(element, i);       findShowMore(res, child);   } }
and than use:
if (show) {   element.getStyle().clearDisplay(); } else {  
element.getStyle().setDisplay(Display.NONE); }