a toplevel window with Qt::WA_PaintOnScreen attribute still need backing store

This code will create backing store for a toplevel window including it has Qt::WA_PaintOnScreen attribute:
// a real toplevel window needs a backing store
if (isWindow() && windowType() != Qt::Desktop)

{ d->topData()->backingStore.destroy(); if (hasBackingStoreSupport()) d->topData()->backingStore.create(this); }

If modify it to:
if (isWindow() && windowType() != Qt::Desktop)

{ d->topData()->backingStore.destroy(); if (hasBackingStoreSupport() && !testAttribute(Qt::WA_PaintOnScreen)) d->topData()->backingStore.create(this); }

Some widgets can't work properly.