StackPanelInfo panel = (StackPanelInfo) frame.getChildrenWidgets().get(0);
WidgetInfo button_1 = panel.getChildrenWidgets().get(0);
WidgetInfo button_2 = panel.getChildrenWidgets().get(1);
WidgetInfo button_3 = panel.getChildrenWidgets().get(2);
// show "button_3"
panel.showWidget(button_3);
assertFalse(isVisible(button_1));
assertFalse(isVisible(button_2));
assertTrue(isVisible(button_3));
// delete "button_3", so "button_1" should be displayed
button_3.delete();