@Override
			public void handleEvent (Event e) {
				int visibleCount = 0;
				Rectangle rect = tree.getClientArea ();
				TreeItem item = tree.getTopItem ();
				while (item != null) {
					visibleCount++;
					Rectangle itemRect = item.getBounds();
					if (itemRect.y + itemRect.height > rect.y + rect.height) {
						break;
					}
					item = nextItem (tree, item);
				}				
				button.setText ("Visible Items [" + visibleCount + "]");
			}