void onScrollVertical (Event event) {
	ScrollBar vBar = getVerticalBar ();
	if (vBar == null) return;
	int newSelection = vBar.getSelection ();
	update ();
	GC gc = new GC (this);
	gc.copyArea (
		0, 0,
		clientArea.width, clientArea.height,
		0, (topIndex - newSelection) * itemHeight);
	gc.dispose ();
	topIndex = newSelection;
}