code: public void historyUpdated (int adjustment)
      {
          if (adjustment != 0) {
              for (int ii = 0, nn = _showingHistory.size(); ii < nn; ii++) {
                  ChatGlyph cg = _showingHistory.get(ii);
                  cg.histIndex -= adjustment;
              }
              // some history entries were deleted, we need to re-figure the history scrollbar action
              resetHistoryOffset();
          }
  
          if (isLaidOut() && isHistoryMode()) {
              int val = _historyModel.getValue();
              updateHistBar(val - adjustment);
  
              // only repaint if we need to
              if ((val != _historyModel.getValue()) || (adjustment != 0) || !_histOffsetFinal) {
                  figureCurrentHistory();
              }
          }
      }
docs: from interface HistoryList.Observer