Make sure we don't delete child QWindows if they have WA_NativeWindow set

Deleting a QWindow automatically deletes its child windows because
they are in the QObject hierarchy. However, if the user sets both
WA_NativeWindow and WA_DontCreateNativeAncestors, we can't just
delete that widget's QWindow. First because the widget doesn't get
notified (and maybe it should be), and then because we may invalidate
any reference to the QWindow the user may have kept.

Our solution is to reparent the child QWindows into the new parent's
closest QWindow. We must, however, take the precaution of not keeping
any reference to the backing store in the platform window. Reparenting
operations can trigger repaints on the platform window, but the backing
store is not set and flushed until later.

Task-number: QTBUG-38377
Change-Id: I353f5528f227a227b6d10419367cbe1d5d07a94e
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>

src/plugins/platforms/cocoa/qnsview.mm

@@ -580,6 +580,8 @@ static NSString *_q_NSWindowDidChangeOcclusionStateNotification = nil;
     CGImageRelease(subMask);
 
     [self invalidateWindowShadowIfNeeded];
+
+    m_backingStore = 0;
 }
 
 - (BOOL) isFlipped

src/widgets/kernel/qwidget_qpa.cpp

@@ -284,6 +284,22 @@ void QWidgetPrivate::setParent_sys(QWidget *newparent, Qt::WindowFlags f)
     if (wasCreated && !(f & Qt::Window) && (oldFlags & Qt::Window) && !q->testAttribute(Qt::WA_NativeWindow)) {
         if (extra && extra->hasWindowContainer)
             QWindowContainer::toplevelAboutToBeDestroyed(q);
+
+        QWindow *newParentWindow = newparent->windowHandle();
+        if (!newParentWindow)
+            if (QWidget *npw = newparent->nativeParentWidget())
+                newParentWindow = npw->windowHandle();
+
+        Q_FOREACH (QObject *child, q->windowHandle()->children()) {
+            QWindow *childWindow = qobject_cast<QWindow *>(child);
+            if (!childWindow)
+                continue;
+
+            QWidgetWindow *childWW = qobject_cast<QWidgetWindow *>(childWindow);
+            QWidget *childWidget = childWW ? childWW->widget() : 0;
+            if (!childWW || (childWidget && childWidget->testAttribute(Qt::WA_NativeWindow)))
+                childWindow->setParent(newParentWindow);
+        }
         q->destroy();
     }