@SubscribeUI(type = PointerEnter)
private void show() {
popup.append(content);
popup.add(PopupViewStyle.Show.class);
ClientRect popupArea = popup.position();
ClientRect targetArea = root.position();
popup.css("top", targetArea.bottom() + 15 + "px")
.css("left", targetArea.left() - popupArea.width() / 2 + targetArea.width() / 2 + "px");
}