final History history = IGV.getInstance().getSession().getHistory();
List<History.Entry> allLoci = IGV.getInstance().getSession().getAllHistory();
boolean hasBack = history.peekBack() != null;
boolean hasForward = history.peekForward() != null;
backItem.setEnabled(hasBack);
forwardItem.setEnabled(hasForward);
clearAllItem.setEnabled(allLoci.size() > 0);