@Override
public void expand() {
assert collapsible : "Calling expand on non-collapsible field set";
if (collapsible) {
if (fireCancellableEvent(new BeforeExpandEvent())) {
this.collapsed = false;
appearance.onCollapse(getElement(), false);
getCollapseButton().changeStyle(ToolButton.UP);
fireEvent(new ExpandEvent());
}