qwidget, disable updatesEnabled, do visual change, enable updatesEnabled does not work on QMainWindow

In document https://doc.qt.io/qt-5/qwidget.html#updatesEnabled-prop, there is below sample code:

setUpdatesEnabled(false);
bigVisualChanges();
setUpdatesEnabled(true);

Used that in QMainWindow, but the visual change does not reflect immediately after setUpdatesEnabled(true); is called. Additionally calling repaint or force a resize can make the visual change appears. Please see attached sample