A widget application typically overrides QWidget::closeEvent() and can ignore() the event to prevent closing the window. The typical use case is to open a file dialog and prompt the user to save any unsaved work. In Qt Quick, one could override QWindow::event(), but there is no way to do it with a QML Window { }; and if you try to use C++ for that, you will need to instantiate your subclass of window rather than a plain QQuickWindow, making it non-transparent in QML. So we need to add a similar mechanism, so that in QML it's possible to write an onClosing(closeEvent) handler or something like that, and reject/ignore the event.