Make QQuickWidget play nicer with the software backend

Launching with QT_QUICK_BACKEND=software should never lead to doing
anything QRhi-related, neither in Qt Quick, neither on the Widgets side.

Fixes: QTBUG-101884
Change-Id: I0dc47e04a0d32904db04f32a1db109029f8f4b44
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

src/quick/items/qquickrendercontrol.cpp

@@ -211,7 +211,12 @@ QQuickRenderControl::~QQuickRenderControl()
 
     delete d->rc;
 
-    d->resetRhi();
+    // Only call rhi related cleanup when we actually got to initialize() and
+    // managed to get a QRhi. The software backend for instance would mean
+    // using the rendercontrol without ever calling initialize() - it is then
+    // important to completely skip calling any QSGRhiSupport functions.
+    if (d->rhi)
+        d->resetRhi();
 }
 
 void QQuickRenderControlPrivate::windowDestroyed()

src/quickwidgets/qquickwidget.cpp

@@ -179,7 +179,8 @@ QWindow *QQuickWidgetRenderControl::renderWindow(QPoint *offset)
 void QQuickWidgetPrivate::initOffscreenWindow()
 {
     Q_Q(QQuickWidget);
-    offscreenWindow = new QQuickWidgetOffscreenWindow(*new QQuickWidgetOffscreenWindowPrivate(), renderControl);
+
+    ensureBackingScene();
     offscreenWindow->setScreen(q->screen());
     // Do not call create() on offscreenWindow.
 
@@ -192,17 +193,30 @@ void QQuickWidgetPrivate::initOffscreenWindow()
 #endif
 }
 
-void QQuickWidgetPrivate::init(QQmlEngine* e)
+void QQuickWidgetPrivate::ensureBackingScene()
 {
-    Q_Q(QQuickWidget);
+    // This should initialize, if not already done, the absolute minimum set of
+    // mandatory backing resources, meaning the QQuickWindow and its
+    // QQuickRenderControl. This function may be called very early on upon
+    // construction, including before init() even.
 
-    renderControl = new QQuickWidgetRenderControl(q);
-    initOffscreenWindow();
+    Q_Q(QQuickWidget);
+    if (!renderControl)
+        renderControl = new QQuickWidgetRenderControl(q);
+    if (!offscreenWindow)
+        offscreenWindow = new QQuickWidgetOffscreenWindow(*new QQuickWidgetOffscreenWindowPrivate(), renderControl);
 
     // Check if the Software Adaptation is being used
     auto sgRendererInterface = offscreenWindow->rendererInterface();
     if (sgRendererInterface && sgRendererInterface->graphicsApi() == QSGRendererInterface::Software)
         useSoftwareRenderer = true;
+}
+
+void QQuickWidgetPrivate::init(QQmlEngine* e)
+{
+    Q_Q(QQuickWidget);
+
+    initOffscreenWindow();
 
     if (!useSoftwareRenderer) {
         if (QGuiApplicationPrivate::platformIntegration()->hasCapability(QPlatformIntegration::RhiBasedRendering))
@@ -1280,6 +1294,7 @@ void QQuickWidgetPrivate::setRootObject(QObject *obj)
 
 QPlatformBackingStoreRhiConfig QQuickWidgetPrivate::rhiConfig() const
 {
+    const_cast<QQuickWidgetPrivate *>(this)->ensureBackingScene();
     if (useSoftwareRenderer)
         return {};
 

src/quickwidgets/qquickwidget_p.h

@@ -104,6 +104,7 @@ public:
     QImage grabFramebuffer() override;
 
     void init(QQmlEngine* e = 0);
+    void ensureBackingScene();
     void initOffscreenWindow();
     void ensureEngine() const;
     void handleWindowChange();

tests/auto/quickwidgets/qquickwidget/tst_qquickwidget.cpp

@@ -127,7 +127,6 @@ public:
     tst_qquickwidget();
 
 private slots:
-    void initTestCase();
     void showHide();
     void reparentAfterShow();
     void changeGeometry();
@@ -163,21 +162,6 @@ private:
 tst_qquickwidget::tst_qquickwidget()
     : QQmlDataTest(QT_QMLTEST_DATADIR)
 {
-    QQuickWindow::setGraphicsApi(QSGRendererInterface::OpenGLRhi);
-}
-
-void tst_qquickwidget::initTestCase()
-{
-    QQmlDataTest::initTestCase();
-
-    // ### QTBUG-101884 Skip this test in the QEMU configs in the CI. That runs
-    // with the offscreen plugin and the software backend of Qt Quick. Until
-    // some of the test cases are investigated, skip the whole test.
-    if ((QGuiApplication::platformName() == QLatin1String("offscreen"))
-        || (QGuiApplication::platformName() == QLatin1String("minimal")))
-    {
-        QSKIP("Skipping due to using a an offscreen platform / software rendering");
-    }
 }
 
 void tst_qquickwidget::showHide()