Parent widget does not delete its backing store when native child widget is made full-screen

QWidget parent;
QWidget child(&parent);
QVBoxLayout layout(&parent);
layout.addWidget(&child);
parent.setLayout(&layout);
child.winId();
parent.show();

// Child shares parent's backing store

// Make child widget full screen
child.setWindowFlags((child.windowFlags() | Qt::Window) ^ Qt::SubWindow);
child.setWindowState(child.windowState() | Qt::WindowFullScreen);
child.show();

// Because child's extent now goes beyond parent's, it gets its own backing store

// However, backing store of (now totally occluded) parent widget is not deleted