Fixed crash when scrolling with deleted backingstore

In Symbian we save memory by deleting widget backingstore from
non-visible windows. 12-key FEP is fullscreen and causes the
backing store to be deleted but still continues to scroll the view
to show current caret position in focused lineedit control.

Task-number: QTBUG-5922
Reviewed-by: Jason Barron

2 files changed, 40 insertions, 0 deletions

diff --git a/src/gui/painting/qbackingstore.cpp b/src/gui/painting/qbackingstore.cpp
index 8226797..1b8f413 100644
--- a/src/gui/painting/qbackingstore.cpp
+++ b/src/gui/painting/qbackingstore.cpp
@@ -949,6 +949,8 @@ void QWidgetPrivate::scrollRect(const QRect &rect, int dx, int dy)
         return;
 
     QWidgetBackingStore *wbs = x->backingStore;
+    if (!wbs)
+        return;
 
     static int accelEnv = -1;
     if (accelEnv == -1) {
diff --git a/tests/auto/qwidget/tst_qwidget.cpp b/tests/auto/qwidget/tst_qwidget.cpp
index 1e3f5f8..9960f47 100644
--- a/tests/auto/qwidget/tst_qwidget.cpp
+++ b/tests/auto/qwidget/tst_qwidget.cpp
@@ -391,6 +391,7 @@ private slots:
 #endif
 
     void focusProxyAndInputMethods();
+    void scrollWithoutBackingStore();
 
 private:
     bool ensureScreenSize(int width, int height);
@@ -9703,5 +9704,42 @@ void tst_QWidget::focusProxyAndInputMethods()
     delete toplevel;
 }
 
+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;
+        }
+    }
+    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())->invalidateBuffer(this->rect());
+            repaint();
+        }
+    }
+};
+
+void tst_QWidget::scrollWithoutBackingStore()
+{
+    scrollWidgetWBS scrollable;
+    scrollable.resize(100,100);
+    QLabel child(QString("@"),&scrollable);
+    child.resize(50,50);
+    scrollable.show();
+    QTest::qWaitForWindowShown(&scrollable);
+    scrollable.scroll(50,50);
+    QCOMPARE(child.pos(),QPoint(50,50));
+    scrollable.deleteBackingStore();
+    scrollable.scroll(-25,-25);
+    QCOMPARE(child.pos(),QPoint(25,25));
+    scrollable.enableBackingStore();
+    QCOMPARE(child.pos(),QPoint(25,25));
+}
+
 QTEST_MAIN(tst_QWidget)
 #include "tst_qwidget.moc"