code: public boolean isFocusable(Element elem) {
      return focusableTypes.contains(elem.getTagName().toLowerCase(Locale.ROOT))
          || elem.getTabIndex() >= 0;
    }
docs: Check if an element is focusable. If an element is focusable, the cell
  widget should not steal focus from it.
  
  @param elem the element
  @return true if the element is focusable, false if not