code: public void setWidgetToggleDisplayAllowed(Widget child, boolean allowed) {
          assertIsChild(child);
          Splitter splitter = getAssociatedSplitter(child);
          // The splitter is null for the center element.
          if (splitter != null) {
              splitter.setToggleDisplayAllowed(allowed);
          }
      }
docs: Sets whether or not double-clicking on the splitter should toggle the
  display of the widget.
  
  @param child the child whose display toggling will be allowed or not.
  @param allowed whether or not display toggling is allowed for this widget