Backing store is not deleted if widget is partially shown, fully shown, then hidden

QWidget upper;
upper.setGeometry(50, 50, 100, 100);

QWidget lower;
lower.setGeometry(lower.geometry());

lower.show();
upper.show();
upper.raise();

// At this point, upper has a backing store but lower does not

// Partially reveal lower
upper.move(100, 100);

// lower receives a PartiallyVisible event, so its reference count goes from 0 -> 1

// Completely reveal lower
upper.hide();

// lower receives a PartiallyVisible event, so its reference count goes from 1 -> 2

lower.hide();

// lower receives a NotVisible event, so its reference count goes from 2 -> 1
// The backing store is therefore not deleted