When a QMainWindow contains a QScrollArea which itself contains a QQuickView inside a window container (QWidget::createWindowContainer), then after closing it for the first time, subsequent show calls will only show the window, but none of the children.
It seems the reason is that no show events are sent to the children of QMainWindow.
Interestingly, it really seems to be the combination of scroll area and window container that causes the problem - when embedding the window container directly within the main window or when using a QQuickWidget instead of the QQuickView + window container everything works fine.
To reproduce it, start the attached sample, then press the "Open popup", close the popup windows and press the "Open popup" button again. You'll notice that when opening the window for the first time, the green QML Rectangle is shown inside the scroll area, while the second time around no widgets - not even the status bar - are shown