Fix moving a hidden QOpenGLWidget to another window

A visibile QOpenGLWidget receives a QEvent::WindowChangeInternal which triggers
a QOpenGLWidget::reset(). A hidden QOpenGLWidget never received this event
so it was never reset, resulting in a black rendering.

Includes unit-test that fails without this patch.

Change-Id: I9d2c57d66fa629f631a9829a5ebf4de09998ad75
Task-Id: QTBUG-60896
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/widgets/kernel/qopenglwidget.cpp

@@ -1342,6 +1342,8 @@ bool QOpenGLWidget::event(QEvent *e)
             break;
         if (d->initialized)
             d->reset();
+        if (isHidden())
+            break;
         // FALLTHROUGH
     case QEvent::Show: // reparenting may not lead to a resize so reinitalize on Show too
         if (!d->initialized && !size().isEmpty() && window() && window()->windowHandle()) {

src/widgets/kernel/qwidget.cpp

@@ -10511,7 +10511,7 @@ static void sendWindowChangeToTextureChildrenRecursively(QWidget *widget)
 
     for (int i = 0; i < d->children.size(); ++i) {
         QWidget *w = qobject_cast<QWidget *>(d->children.at(i));
-        if (w && !w->isWindow() && !w->isHidden() && QWidgetPrivate::get(w)->textureChildSeen)
+        if (w && !w->isWindow() && QWidgetPrivate::get(w)->textureChildSeen)
             sendWindowChangeToTextureChildrenRecursively(w);
     }
 }

tests/auto/widgets/widgets/qopenglwidget/tst_qopenglwidget.cpp

@@ -55,6 +55,7 @@ private slots:
     void painter();
     void reparentToAlreadyCreated();
     void reparentToNotYetCreated();
+    void reparentHidden();
     void asViewport();
     void requestUpdate();
     void fboRedirect();
@@ -279,6 +280,36 @@ void tst_QOpenGLWidget::reparentToNotYetCreated()
     QVERIFY(image.pixel(20, 10) == qRgb(0, 0, 255));
 }
 
+void tst_QOpenGLWidget::reparentHidden()
+{
+    // Tests QTBUG-60896
+    QWidget topLevel1;
+
+    QWidget *container = new QWidget(&topLevel1);
+    PainterWidget *glw = new PainterWidget(container);
+    topLevel1.resize(640, 480);
+    glw->resize(320, 200);
+    topLevel1.show();
+
+    glw->hide(); // Explicitly hidden
+
+    QTest::qWaitForWindowExposed(&topLevel1);
+
+    QWidget topLevel2;
+    topLevel2.resize(640, 480);
+    topLevel2.show();
+    QTest::qWaitForWindowExposed(&topLevel2);
+
+    QOpenGLContext *originalContext = glw->context();
+    QVERIFY(originalContext);
+
+    container->setParent(&topLevel2);
+    glw->show(); // Should get a new context now
+
+    QOpenGLContext *newContext = glw->context();
+    QVERIFY(originalContext != newContext);
+}
+
 class CountingGraphicsView : public QGraphicsView
 {
 public: