Fix issue where revealed widget children do not receive paint event.

When a child of a widget is spontaneously revealed due to a call to
the parent 'resize' method, the child will not receive a paint event
if it has the WA_StaticContents and WA_OpaquePaintEvent flags set.

This is caused by the backing store being pre-emptively resized by the
call to setGeometry_sys, which causes QWidgetBackingStore::sync to skip
the block which handles the static contents.

There doesn't appear to be any reason to preemptively resize the backing
store, since it is always resized as-needed during the the 'sync' method.
This change-set removes the code which preemptively resizes the backing
store.

Task-number: QTBUG-35282
Change-Id: Ie9942854ca5322dfe0f98ed8100810161576be80
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/widgets/kernel/qwidget_qpa.cpp

@@ -549,12 +549,6 @@ void QWidgetPrivate::show_sys()
                 window->resize(geomRect.size());
         }
 
-        if (QBackingStore *store = q->backingStore()) {
-            if (store->size() != geomRect.size()) {
-                store->resize(geomRect.size());
-            }
-        }
-
 #ifndef QT_NO_CURSOR
         qt_qpa_set_cursor(q, false); // Needed in case cursor was set before show
 #endif
@@ -762,11 +756,6 @@ void QWidgetPrivate::setGeometry_sys(int x, int y, int w, int h, bool isMove)
                     QPoint posInNativeParent =  q->mapTo(q->nativeParentWidget(),QPoint());
                     q->windowHandle()->setGeometry(QRect(posInNativeParent,r.size()));
                 }
-                const QWidgetBackingStore *bs = maybeBackingStore();
-                if (bs && bs->store) {
-                    if (isResize)
-                        bs->store->resize(r.size());
-                }
 
                 if (needsShow)
                     show_sys();

src/widgets/kernel/qwidgetbackingstore.cpp

@@ -940,7 +940,7 @@ void QWidgetBackingStore::sync(QWidget *exposedWidget, const QRegion &exposedReg
     }
 
     // Nothing to repaint.
-    if (!isDirty()) {
+    if (!isDirty() && store->size().isValid()) {
         qt_flush(exposedWidget, exposedRegion, store, tlw, tlwOffset, widgetTextures);
         return;
     }
@@ -1035,7 +1035,7 @@ void QWidgetBackingStore::doSync()
     const QRect tlwRect(topLevelRect());
     const QRect surfaceGeometry(tlwRect.topLeft(), store->size());
     if ((fullUpdatePending || inTopLevelResize || surfaceGeometry.size() != tlwRect.size()) && !updatesDisabled) {
-        if (hasStaticContents()) {
+        if (hasStaticContents() && !store->size().isEmpty() ) {
             // Repaint existing dirty area and newly visible area.
             const QRect clipRect(0, 0, surfaceGeometry.width(), surfaceGeometry.height());
             const QRegion staticRegion(staticContents(0, clipRect));

tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp

@@ -441,6 +441,8 @@ private slots:
     void mouseDoubleClickBubbling_QTBUG29680();
     void largerThanScreen_QTBUG30142();
 
+    void resizeStaticContentsChildWidget_QTBUG35282();
+
 private:
     bool ensureScreenSize(int width, int height);
     QWidget *testWidget;
@@ -10220,5 +10222,26 @@ void tst_QWidget::largerThanScreen_QTBUG30142()
     QVERIFY(widget2.frameGeometry().x() >= 0);
 }
 
+void tst_QWidget::resizeStaticContentsChildWidget_QTBUG35282()
+{
+    QWidget widget;
+    widget.resize(200,200);
+
+    UpdateWidget childWidget(&widget);
+    childWidget.setAttribute(Qt::WA_StaticContents);
+    childWidget.setAttribute(Qt::WA_OpaquePaintEvent);
+    childWidget.setGeometry(250, 250, 500, 500);
+
+    widget.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&widget));
+    QVERIFY(childWidget.numPaintEvents == 0);
+    childWidget.reset();
+
+    widget.resize(1000,1000);
+    QVERIFY(QTest::qWaitForWindowExposed(&widget));
+    QGuiApplication::sync();
+    QVERIFY(childWidget.numPaintEvents >= 1);
+}
+
 QTEST_MAIN(tst_QWidget)
 #include "tst_qwidget.moc"