Child widget does not delete its backing store when fullscreen is disabled

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

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

// Child's backing store should be deleted, but it is not