Merge branch '4.6' of scm.dev.nokia.troll.no:qt/qt-s60-public into 4.6-integration

* '4.6' of scm.dev.nokia.troll.no:qt/qt-s60-public:
  Removed compiler warnings
  Window visibility changes update TLW backing store reference count
  Enable visibility change events on all Symbian native windows
  Added reference counting to QWidgetBackingStore
  Added tst_QWidget::destroyBackingStoreWhenHidden

14 files changed, 377 insertions, 35 deletions

diff --git a/src/3rdparty/phonon/mmf/abstractaudioeffect.cpp b/src/3rdparty/phonon/mmf/abstractaudioeffect.cpp
index 6cfeb768ae..9b64ab3ff2 100644
--- a/src/3rdparty/phonon/mmf/abstractaudioeffect.cpp
+++ b/src/3rdparty/phonon/mmf/abstractaudioeffect.cpp
@@ -184,6 +184,8 @@ int AbstractAudioEffect::effectParameterChanged(
     const EffectParameter &param, const QVariant &value)
 {
     // Default implementation
+    Q_UNUSED(param)
+    Q_UNUSED(value)
     Q_ASSERT_X(false, Q_FUNC_INFO, "Effect has no parameters");
     return 0;
 }
diff --git a/src/3rdparty/phonon/mmf/abstractaudioeffect.h b/src/3rdparty/phonon/mmf/abstractaudioeffect.h
index d71993b146..70adcf644f 100644
--- a/src/3rdparty/phonon/mmf/abstractaudioeffect.h
+++ b/src/3rdparty/phonon/mmf/abstractaudioeffect.h
@@ -118,6 +118,10 @@ private:
 // for each of the effects.  Using this reduces repetition of boilerplate
 // in the implementations of the backend effect nodes.
 
+#ifdef Q_CC_NOKIAX86
+#   pragma warn_illtokenpasting off
+#endif
+
 #define PHONON_MMF_DEFINE_EFFECT_FUNCTIONS(Effect)                      \
                                                                         \
 void Effect##::createEffect(AudioPlayer::NativePlayer *player)          \
diff --git a/src/3rdparty/phonon/mmf/audiooutput.cpp b/src/3rdparty/phonon/mmf/audiooutput.cpp
index c6be20b3de..358868379b 100644
--- a/src/3rdparty/phonon/mmf/audiooutput.cpp
+++ b/src/3rdparty/phonon/mmf/audiooutput.cpp
@@ -78,6 +78,9 @@ bool MMF::AudioOutput::setOutputDevice(int index)
 {
     Q_ASSERT_X(index == AudioOutputDeviceID, Q_FUNC_INFO,
                "We only support one output device, with id 0");
+#ifdef QT_NO_DEBUG
+    Q_UNUSED(index)
+#endif
     return true;
 }
 
diff --git a/src/3rdparty/phonon/mmf/bassboost.cpp b/src/3rdparty/phonon/mmf/bassboost.cpp
index c7af93924b..67076f62d0 100644
--- a/src/3rdparty/phonon/mmf/bassboost.cpp
+++ b/src/3rdparty/phonon/mmf/bassboost.cpp
@@ -49,6 +49,7 @@ const char* BassBoost::description()
 bool BassBoost::getParameters(CMdaAudioOutputStream *stream,
     QList<EffectParameter> &parameters)
 {
+    Q_UNUSED(parameters)
     QScopedPointer<CBassBoost> effect;
     TRAPD(err, effect.reset(CBassBoost::NewL(*stream)));
     return (KErrNone == err);
diff --git a/src/3rdparty/phonon/mmf/effectfactory.cpp b/src/3rdparty/phonon/mmf/effectfactory.cpp
index c5e33d5c7a..4643978d2f 100644
--- a/src/3rdparty/phonon/mmf/effectfactory.cpp
+++ b/src/3rdparty/phonon/mmf/effectfactory.cpp
@@ -183,8 +183,9 @@ EffectFactory::EffectData EffectFactory::getData()
          /* defaultValue */       QVariant(bool(true)));
     data.m_parameters.append(param);
 
-    if (data.m_supported = BackendNode::getParameters
-            (stream.data(), data.m_parameters)) {
+    data.m_supported = BackendNode::getParameters(stream.data(),
+                                                  data.m_parameters);
+    if (data.m_supported) {
         const QString description = QCoreApplication::translate
             ("Phonon::MMF::EffectFactory", BackendNode::description());
         data.m_descriptions.insert("name", description);
diff --git a/src/3rdparty/phonon/mmf/loudness.cpp b/src/3rdparty/phonon/mmf/loudness.cpp
index 1079a35653..ca05ab0989 100644
--- a/src/3rdparty/phonon/mmf/loudness.cpp
+++ b/src/3rdparty/phonon/mmf/loudness.cpp
@@ -49,6 +49,7 @@ const char* Loudness::description()
 bool Loudness::getParameters(CMdaAudioOutputStream *stream,
     QList<EffectParameter> &parameters)
 {
+    Q_UNUSED(parameters)
     QScopedPointer<CLoudness> effect;
     TRAPD(err, effect.reset(CLoudness::NewL(*stream)));
     return (KErrNone == err);
diff --git a/src/gui/embedded/qwsmanager_qws.cpp b/src/gui/embedded/qwsmanager_qws.cpp
index d6ef14858b..ac6d36eb25 100644
--- a/src/gui/embedded/qwsmanager_qws.cpp
+++ b/src/gui/embedded/qwsmanager_qws.cpp
@@ -392,7 +392,7 @@ void QWSManagerPrivate::dirtyRegion(int decorationRegion,
                                     const QRegion &clip)
 {
     QTLWExtra *topextra = managed->d_func()->extra->topextra;
-    QWidgetBackingStore *bs = topextra->backingStore;
+    QWidgetBackingStore *bs = topextra->backingStore.data();
     const bool pendingUpdateRequest = bs->isDirty();
 
     if (decorationRegion == QDecoration::All) {
diff --git a/src/gui/kernel/qapplication_s60.cpp b/src/gui/kernel/qapplication_s60.cpp
index b1e0f63d0f..78027b2752 100644
--- a/src/gui/kernel/qapplication_s60.cpp
+++ b/src/gui/kernel/qapplication_s60.cpp
@@ -1703,19 +1703,27 @@ int QApplicationPrivate::symbianProcessWsEvent(const QSymbianEvent *symbianEvent
                 return 1;
             const TWsVisibilityChangedEvent *visChangedEvent = event->VisibilityChanged();
             QWidget *w = QWidgetPrivate::mapper->value(control);
-            if (!w->d_func()->maybeTopData())
+            QWidget *const window = w->window();
+            if (!window->d_func()->maybeTopData())
                 break;
+            QRefCountedWidgetBackingStore &backingStore = window->d_func()->maybeTopData()->backingStore;
             if (visChangedEvent->iFlags & TWsVisibilityChangedEvent::ENotVisible) {
-                delete w->d_func()->topData()->backingStore;
-                w->d_func()->topData()->backingStore = 0;
+                // Decrement backing store reference count
+                backingStore.deref();
                 // In order to ensure that any resources used by the window surface
                 // are immediately freed, we flush the WSERV command buffer.
                 S60->wsSession().Flush();
-            } else if ((visChangedEvent->iFlags & TWsVisibilityChangedEvent::EPartiallyVisible)
-                       && !w->d_func()->maybeBackingStore()) {
-                w->d_func()->topData()->backingStore = new QWidgetBackingStore(w);
-                w->d_func()->invalidateBuffer(w->rect());
-                w->repaint();
+            } else if (visChangedEvent->iFlags & TWsVisibilityChangedEvent::EPartiallyVisible) {
+                if (backingStore.data()) {
+                    // Increment backing store reference count
+                    backingStore.ref();
+                } else {
+                    // Create backing store with an initial reference count of 1
+                    backingStore.create(window);
+                    backingStore.ref();
+                    w->d_func()->invalidateBuffer(w->rect());
+                    w->repaint();
+                }
             }
             return 1;
         }
diff --git a/src/gui/kernel/qapplication_x11.cpp b/src/gui/kernel/qapplication_x11.cpp
index 25a7750d96..304cd173e9 100644
--- a/src/gui/kernel/qapplication_x11.cpp
+++ b/src/gui/kernel/qapplication_x11.cpp
@@ -5245,7 +5245,7 @@ bool QETWidget::translateConfigEvent(const XEvent *event)
 
         if (isVisible() && data->crect.size() != oldSize) {
             Q_ASSERT(d->extra->topextra);
-            QWidgetBackingStore *bs = d->extra->topextra->backingStore;
+            QWidgetBackingStore *bs = d->extra->topextra->backingStore.data();
             const bool hasStaticContents = bs && bs->hasStaticContents();
             // If we have a backing store with static contents, we have to disable the top-level
             // resize optimization in order to get invalidated regions for resized widgets.
diff --git a/src/gui/kernel/qwidget.cpp b/src/gui/kernel/qwidget.cpp
index e180001291..a9ea9ac563 100644
--- a/src/gui/kernel/qwidget.cpp
+++ b/src/gui/kernel/qwidget.cpp
@@ -161,6 +161,51 @@ static inline bool hasBackingStoreSupport()
 extern bool qt_sendSpontaneousEvent(QObject*, QEvent*); // qapplication.cpp
 extern QDesktopWidget *qt_desktopWidget; // qapplication.cpp
 
+
+QRefCountedWidgetBackingStore::QRefCountedWidgetBackingStore()
+    :   m_ptr(0)
+    ,   m_count(0)
+{
+
+}
+
+QRefCountedWidgetBackingStore::~QRefCountedWidgetBackingStore()
+{
+    delete m_ptr;
+}
+
+void QRefCountedWidgetBackingStore::create(QWidget *widget)
+{
+    destroy();
+    m_ptr = new QWidgetBackingStore(widget);
+    m_count = 0;
+}
+
+void QRefCountedWidgetBackingStore::destroy()
+{
+    delete m_ptr;
+    m_ptr = 0;
+    m_count = 0;
+}
+
+void QRefCountedWidgetBackingStore::ref()
+{
+    Q_ASSERT(m_ptr);
+    ++m_count;
+}
+
+void QRefCountedWidgetBackingStore::deref()
+{
+    if (m_count) {
+        Q_ASSERT(m_ptr);
+        if (0 == --m_count) {
+            delete m_ptr;
+            m_ptr = 0;
+        }
+    }
+}
+
+
 QWidgetPrivate::QWidgetPrivate(int version)
     : QObjectPrivate(version)
       , extra(0)
@@ -1324,11 +1369,9 @@ void QWidget::create(WId window, bool initializeWindow, bool destroyOldWindow)
 
     // a real toplevel window needs a backing store
     if (isWindow() && windowType() != Qt::Desktop) {
-        delete d->topData()->backingStore;
-        // QWidgetBackingStore will check this variable, hence it must be 0
-        d->topData()->backingStore = 0;
+        d->topData()->backingStore.destroy();
         if (hasBackingStoreSupport())
-            d->topData()->backingStore = new QWidgetBackingStore(this);
+            d->topData()->backingStore.create(this);
     }
 
     d->setModal_sys();
@@ -1451,8 +1494,7 @@ QWidget::~QWidget()
         // the backing store will delete its window surface, which may or may
         // not have a reference to this widget that will be used later to
         // notify the window it no longer has a surface.
-        delete d->extra->topextra->backingStore;
-        d->extra->topextra->backingStore = 0;
+        d->extra->topextra->backingStore.destroy();
     }
 #endif
     if (QWidgetBackingStore *bs = d->maybeBackingStore()) {
@@ -1540,7 +1582,6 @@ void QWidgetPrivate::createTLExtra()
         QTLWExtra* x = extra->topextra = new QTLWExtra;
         x->icon = 0;
         x->iconPixmap = 0;
-        x->backingStore = 0;
         x->windowSurface = 0;
         x->sharedPainter = 0;
         x->incw = x->inch = 0;
@@ -1619,7 +1660,7 @@ void QWidgetPrivate::deleteExtra()
 #endif
         if (extra->topextra) {
             deleteTLSysExtra();
-            delete extra->topextra->backingStore;
+            extra->topextra->backingStore.destroy();
             delete extra->topextra->icon;
             delete extra->topextra->iconPixmap;
 #if defined(Q_WS_QWS) && !defined(QT_NO_QWS_MANAGER)
diff --git a/src/gui/kernel/qwidget_p.h b/src/gui/kernel/qwidget_p.h
index 555647c1df..9f7ad95aac 100644
--- a/src/gui/kernel/qwidget_p.h
+++ b/src/gui/kernel/qwidget_p.h
@@ -110,13 +110,53 @@ class QWidgetItemV2;
 
 class QStyle;
 
+class Q_AUTOTEST_EXPORT QRefCountedWidgetBackingStore
+{
+public:
+    QRefCountedWidgetBackingStore();
+    ~QRefCountedWidgetBackingStore();
+
+    void create(QWidget *tlw);
+    void destroy();
+
+    void ref();
+    void deref();
+
+    inline QWidgetBackingStore* data()
+    {
+        return m_ptr;
+    }
+
+    inline QWidgetBackingStore* operator->()
+    {
+        return m_ptr;
+    }
+
+    inline QWidgetBackingStore& operator*()
+    {
+        return *m_ptr;
+    }
+
+    inline operator bool() const
+    {
+        return (0 != m_ptr);
+    }
+
+private:
+    Q_DISABLE_COPY(QRefCountedWidgetBackingStore)
+
+private:
+    QWidgetBackingStore* m_ptr;
+    int m_count;
+};
+
 struct QTLWExtra {
     // *************************** Cross-platform variables *****************************
 
     // Regular pointers (keep them together to avoid gaps on 64 bits architectures).
     QIcon *icon; // widget icon
     QPixmap *iconPixmap;
-    QWidgetBackingStore *backingStore;
+    QRefCountedWidgetBackingStore backingStore;
     QWindowSurface *windowSurface;
     QPainter *sharedPainter;
 
@@ -907,7 +947,7 @@ inline QWidgetBackingStore *QWidgetPrivate::maybeBackingStore() const
 {
     Q_Q(const QWidget);
     QTLWExtra *x = q->window()->d_func()->maybeTopData();
-    return x ? x->backingStore : 0;
+    return x ? x->backingStore.data() : 0;
 }
 
 QT_END_NAMESPACE
diff --git a/src/gui/kernel/qwidget_s60.cpp b/src/gui/kernel/qwidget_s60.cpp
index 438c2a303a..86b858dabd 100644
--- a/src/gui/kernel/qwidget_s60.cpp
+++ b/src/gui/kernel/qwidget_s60.cpp
@@ -433,6 +433,7 @@ void QWidgetPrivate::create_sys(WId window, bool /* initializeWindow */, bool de
         // Request mouse move events.
         drawableWindow->PointerFilter(EPointerFilterEnterExit
             | EPointerFilterMove | EPointerFilterDrag, 0);
+        drawableWindow->EnableVisibilityChangeEvents();
 
         if (q->isVisible() && q->testAttribute(Qt::WA_Mapped)) {
             activateSymbianWindow(control.data());
@@ -908,14 +909,12 @@ void QWidgetPrivate::registerDropSite(bool /* on */)
 
 void QWidgetPrivate::createTLSysExtra()
 {
-    extra->topextra->backingStore = 0;
     extra->topextra->inExpose = 0;
 }
 
 void QWidgetPrivate::deleteTLSysExtra()
 {
-    delete extra->topextra->backingStore;
-    extra->topextra->backingStore = 0;
+    extra->topextra->backingStore.destroy();
 }
 
 void QWidgetPrivate::createSysExtra()
diff --git a/src/gui/painting/qbackingstore.cpp b/src/gui/painting/qbackingstore.cpp
index 8de9eaa5c8..3ddffef7c2 100644
--- a/src/gui/painting/qbackingstore.cpp
+++ b/src/gui/painting/qbackingstore.cpp
@@ -914,7 +914,7 @@ void QWidgetPrivate::moveRect(const QRect &rect, int dx, int dy)
         invalidateBuffer((newRect & clipR).translated(-data.crect.topLeft()));
     } else {
 
-        QWidgetBackingStore *wbs = x->backingStore;
+        QWidgetBackingStore *wbs = x->backingStore.data();
         QRegion childExpose(newRect & clipR);
 
         if (sourceRect.isValid() && wbs->bltRect(sourceRect, dx, dy, pw))
@@ -957,7 +957,7 @@ void QWidgetPrivate::scrollRect(const QRect &rect, int dx, int dy)
     if (x->inTopLevelResize)
         return;
 
-    QWidgetBackingStore *wbs = x->backingStore;
+    QWidgetBackingStore *wbs = x->backingStore.data();
     if (!wbs)
         return;
 
diff --git a/tests/auto/qwidget/tst_qwidget.cpp b/tests/auto/qwidget/tst_qwidget.cpp
index 76e20b997e..7d4f8ffa1a 100644
--- a/tests/auto/qwidget/tst_qwidget.cpp
+++ b/tests/auto/qwidget/tst_qwidget.cpp
@@ -385,6 +385,7 @@ private slots:
     void setGraphicsEffect();
 
     void destroyBackingStore();
+    void destroyBackingStoreWhenHidden();
 
     void activateWindow();
 
@@ -9499,9 +9500,7 @@ void tst_QWidget::destroyBackingStore()
     QTRY_VERIFY(w.numPaintEvents > 0);
     w.reset();
     w.update();
-    delete qt_widget_private(&w)->topData()->backingStore;
-    qt_widget_private(&w)->topData()->backingStore = 0;
-    qt_widget_private(&w)->topData()->backingStore = new QWidgetBackingStore(&w);
+    qt_widget_private(&w)->topData()->backingStore.create(&w);
 
     w.update();
     QApplication::processEvents();
@@ -9517,6 +9516,252 @@ void tst_QWidget::destroyBackingStore()
 #endif
 }
 
+// Helper function
+QWidgetBackingStore* backingStore(QWidget &widget)
+{
+    QWidgetBackingStore *backingStore = 0;
+#ifdef QT_BUILD_INTERNAL
+    if (QTLWExtra *topExtra = qt_widget_private(&widget)->maybeTopData())
+        backingStore = topExtra->backingStore.data();
+#endif
+    return backingStore;
+}
+
+// Wait for a condition to be true, timing out after 1 second
+// This is used following calls to QWidget::show() and QWidget::hide(), which are
+// expected to asynchronously trigger native window visibility events.
+#define WAIT_AND_VERIFY(condition)                                              \
+    do {                                                                        \
+        QTime start = QTime::currentTime();                                     \
+        while (!(condition) && (start.elapsed() < 1000)) {                      \
+            qApp->processEvents();                                              \
+            QTest::qWait(50);                                                   \
+        }                                                                       \
+        if (!QTest::qVerify((condition), #condition, "", __FILE__, __LINE__))   \
+            return;                                                             \
+    } while (0)
+
+void tst_QWidget::destroyBackingStoreWhenHidden()
+{
+#ifndef QT_BUILD_INTERNAL
+    QSKIP("Test step requires access to Q_AUTOTEST_EXPORT", SkipAll);
+#endif
+
+#ifndef Q_OS_SYMBIAN
+    QSKIP("Only Symbian destroys backing store when native window becomes invisible", SkipAll);
+#endif
+
+    testWidget->hide();
+    QTest::qWait(1000);
+
+    // 1. Single top-level QWidget
+    {
+    QWidget w;
+    w.setAutoFillBackground(true);
+    w.setPalette(Qt::yellow);
+    w.setGeometry(0, 0, 100, 100);
+    w.show();
+    QTest::qWaitForWindowShown(&w);
+    QVERIFY(0 != backingStore(w));
+
+    w.hide();
+    WAIT_AND_VERIFY(0 == backingStore(w));
+
+    w.show();
+    QTest::qWaitForWindowShown(&w);
+    QVERIFY(0 != backingStore(w));
+    }
+
+    // 2. Two top-level widgets
+    {
+    QWidget w1;
+    w1.setGeometry(0, 0, 100, 100);
+    w1.setAutoFillBackground(true);
+    w1.setPalette(Qt::red);
+    w1.show();
+    QTest::qWaitForWindowShown(&w1);
+    QVERIFY(0 != backingStore(w1));
+
+    QWidget w2;
+    w2.setGeometry(w1.geometry());
+    w1.setAutoFillBackground(true);
+    w1.setPalette(Qt::blue);
+    w2.show();
+    QTest::qWaitForWindowShown(&w2);
+    QVERIFY(0 != backingStore(w2));
+
+    // Check that w1 deleted its backing store when obscured by w2
+    QVERIFY(0 == backingStore(w1));
+
+    w2.move(w2.pos() + QPoint(10, 10));
+
+    // Check that w1 recreates its backing store when partially revealed
+    WAIT_AND_VERIFY(0 != backingStore(w1));
+    }
+
+    // 3. Native child widget
+    {
+    QWidget parent;
+    parent.setGeometry(0, 0, 100, 100);
+    parent.setAutoFillBackground(true);
+    parent.setPalette(Qt::yellow);
+
+    QWidget child(&parent);
+    child.setAutoFillBackground(true);
+    child.setPalette(Qt::green);
+
+    QVBoxLayout layout(&parent);
+    layout.setContentsMargins(10, 10, 10, 10);
+    layout.addWidget(&child);
+    parent.setLayout(&layout);
+
+    child.winId();
+
+    parent.show();
+    QTest::qWaitForWindowShown(&parent);
+
+    // Check that child window does not obscure parent window
+    QVERIFY(!parent.visibleRegion().subtracted(child.visibleRegion()).isEmpty());
+
+    // Native child widget should share parent's backing store
+    QWidgetBackingStore *const parentBs = backingStore(parent);
+    QVERIFY(0 != parentBs);
+    QVERIFY(0 == backingStore(child));
+
+    // Set margins to zero so that child widget totally obscures parent
+    layout.setContentsMargins(0, 0, 0, 0);
+
+    WAIT_AND_VERIFY(parent.visibleRegion().subtracted(child.visibleRegion()).isEmpty());
+
+    // Backing store should remain unchanged despite child window obscuring
+    // parent window
+    QVERIFY(parentBs == backingStore(parent));
+    QVERIFY(0 == backingStore(child));
+    }
+
+    // 4. Alien child widget which is made full-screen
+    {
+    QWidget parent;
+    parent.setGeometry(0, 0, 100, 100);
+    parent.setAutoFillBackground(true);
+    parent.setPalette(Qt::red);
+
+    QWidget child(&parent);
+    child.setAutoFillBackground(true);
+    child.setPalette(Qt::blue);
+
+    QVBoxLayout layout(&parent);
+    layout.setContentsMargins(10, 10, 10, 10);
+    layout.addWidget(&child);
+    parent.setLayout(&layout);
+
+    parent.show();
+    QTest::qWaitForWindowShown(&parent);
+
+    // Check that child window does not obscure parent window
+    QVERIFY(!parent.visibleRegion().subtracted(child.visibleRegion()).isEmpty());
+
+    // Native child widget should share parent's backing store
+    QVERIFY(0 != backingStore(parent));
+    QVERIFY(0 == backingStore(child));
+
+    // Make child widget full screen
+    child.setWindowFlags((child.windowFlags() | Qt::Window) ^ Qt::SubWindow);
+    child.setWindowState(child.windowState() | Qt::WindowFullScreen);
+    child.show();
+    QTest::qWaitForWindowShown(&child);
+
+    // Check that child window obscures parent window
+    QVERIFY(parent.visibleRegion().subtracted(child.visibleRegion()).isEmpty());
+
+    // Now that extent of child widget goes beyond parent's extent,
+    // a new backing store should be created for the child widget.
+    QVERIFY(0 != backingStore(child));
+
+    // Parent is obscured, therefore its backing store should be destroyed
+    QVERIFY(0 == backingStore(parent));
+
+    // Disable full screen
+    child.setWindowFlags(child.windowFlags() ^ (Qt::Window | Qt::SubWindow));
+    child.setWindowState(child.windowState() ^ Qt::WindowFullScreen);
+    child.show();
+    QTest::qWaitForWindowShown(&child);
+
+    // Check that parent is now visible again
+    QVERIFY(!parent.visibleRegion().subtracted(child.visibleRegion()).isEmpty());
+
+    // Native child widget should once again share parent's backing store
+    QVERIFY(0 != backingStore(parent));
+    QEXPECT_FAIL("", "QTBUG-10643", Continue);
+    QVERIFY(0 == backingStore(child));
+    }
+
+    // 5. Native child widget which is made full-screen
+    {
+    QWidget parent;
+    parent.setGeometry(0, 0, 100, 100);
+    parent.setAutoFillBackground(true);
+    parent.setPalette(Qt::red);
+
+    QWidget child(&parent);
+    child.setAutoFillBackground(true);
+    child.setPalette(Qt::blue);
+
+    QVBoxLayout layout(&parent);
+    layout.setContentsMargins(10, 10, 10, 10);
+    layout.addWidget(&child);
+    parent.setLayout(&layout);
+
+    child.winId();
+
+    parent.show();
+    QTest::qWaitForWindowShown(&parent);
+
+    // Check that child window does not obscure parent window
+    QVERIFY(!parent.visibleRegion().subtracted(child.visibleRegion()).isEmpty());
+
+    // Native child widget should share parent's backing store
+    QVERIFY(0 != backingStore(parent));
+    QVERIFY(0 == backingStore(child));
+
+    // Make child widget full screen
+    child.setWindowFlags((child.windowFlags() | Qt::Window) ^ Qt::SubWindow);
+    child.setWindowState(child.windowState() | Qt::WindowFullScreen);
+    child.show();
+    QTest::qWaitForWindowShown(&child);
+
+    // Ensure that 'window hidden' event is received by parent
+    qApp->processEvents();
+
+    // Check that child window obscures parent window
+    QVERIFY(parent.visibleRegion().subtracted(child.visibleRegion()).isEmpty());
+
+    // Now that extent of child widget goes beyond parent's extent,
+    // a new backing store should be created for the child widget.
+    QVERIFY(0 != backingStore(child));
+
+    // Parent is obscured, therefore its backing store should be destroyed
+    QEXPECT_FAIL("", "QTBUG-10643", Continue);
+    QVERIFY(0 == backingStore(parent));
+
+    // Disable full screen
+    child.setWindowFlags(child.windowFlags() ^ (Qt::Window | Qt::SubWindow));
+    child.setWindowState(child.windowState() ^ Qt::WindowFullScreen);
+    child.show();
+    QTest::qWaitForWindowShown(&child);
+
+    // Check that parent is now visible again
+    QVERIFY(!parent.visibleRegion().subtracted(child.visibleRegion()).isEmpty());
+
+    // Native child widget should once again share parent's backing store
+    QVERIFY(0 != backingStore(parent));
+    QEXPECT_FAIL("", "QTBUG-10643", Continue);
+    QVERIFY(0 == backingStore(child));
+    }
+}
+
+#undef WAIT_AND_VERIFY
+
 void tst_QWidget::rectOutsideCoordinatesLimit_task144779()
 {
 #ifdef Q_OS_WINCE_WM
@@ -9997,15 +10242,12 @@ class scrollWidgetWBS : public QWidget
 public:
     void deleteBackingStore()
     {
-        if (static_cast<QWidgetPrivate*>(d_ptr.data())->maybeBackingStore()) {
-            delete static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStore;
-            static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStore = 0;
-        }
+        static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStore.destroy();
     }
     void enableBackingStore()
     {
         if (!static_cast<QWidgetPrivate*>(d_ptr.data())->maybeBackingStore()) {
-            static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStore = new QWidgetBackingStore(this);
+            static_cast<QWidgetPrivate*>(d_ptr.data())->topData()->backingStore.create(this);
             static_cast<QWidgetPrivate*>(d_ptr.data())->invalidateBuffer(this->rect());
             repaint();
         }