Expose screen change event to widgets so that they can check the dpr

Unlike ordinary widgets, the ones that have OpenGL framebuffers must know
about screen changes because the device pixel ratio may be different on
the new screen. Add an internal event, ScreenChangeInternal, as the
counterpart to WindowChangeInternal.

Change-Id: I5e55999838e4c0284e7d9832299f7cc6b541ee3f
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

src/corelib/kernel/qcoreevent.cpp

@@ -278,6 +278,7 @@ QT_BEGIN_NAMESPACE
     \omitvalue FutureCallOut
     \omitvalue NativeGesture
     \omitvalue WindowChangeInternal
+    \omitvalue ScreenChangeInternal
 */
 
 /*!

src/corelib/kernel/qcoreevent.h

@@ -281,6 +281,7 @@ public:
         ApplicationStateChange = 214,
 
         WindowChangeInternal = 215,             // internal for QQuickWidget
+        ScreenChangeInternal = 216,
 
         // 512 reserved for Qt Jambi's MetaCall event
         // 513 reserved for Qt Jambi's DeleteOnMainThread event

src/widgets/kernel/qopenglwidget.cpp

@@ -1061,6 +1061,10 @@ bool QOpenGLWidget::event(QEvent *e)
                 d->recreateFbo();
         }
         break;
+    case QEvent::ScreenChangeInternal:
+        if (d->initialized && d->paintDevice->devicePixelRatio() != devicePixelRatio())
+            d->recreateFbo();
+        break;
     default:
         break;
     }

src/widgets/kernel/qwidgetwindow.cpp

@@ -99,7 +99,7 @@ QWidgetWindow::QWidgetWindow(QWidget *widget)
         setSurfaceType(QSurface::RasterGLSurface);
     }
     connect(m_widget, &QObject::objectNameChanged, this, &QWidgetWindow::updateObjectName);
-    connect(this, SIGNAL(screenChanged(QScreen*)), this, SLOT(repaintWindow()));
+    connect(this, SIGNAL(screenChanged(QScreen*)), this, SLOT(handleScreenChange()));
 }
 
 QWidgetWindow::~QWidgetWindow()
@@ -558,8 +558,27 @@ void QWidgetWindow::updateGeometry()
     m_widget->data->fstrut_dirty = false;
 }
 
-// Invalidates the backing store buffer and repaints immediately.
-// ### Qt 5.4: replace with QUpdateWindowRequestEvent.
+static void sendScreenChangeRecursively(QWidget *widget)
+{
+    QEvent e(QEvent::ScreenChangeInternal);
+    QApplication::sendEvent(widget, &e);
+    QWidgetPrivate *d = QWidgetPrivate::get(widget);
+    for (int i = 0; i < d->children.size(); ++i) {
+        QWidget *w = qobject_cast<QWidget *>(d->children.at(i));
+        if (w)
+            sendScreenChangeRecursively(w);
+    }
+}
+
+void QWidgetWindow::handleScreenChange()
+{
+    // Send an event recursively to the widget and its children.
+    sendScreenChangeRecursively(m_widget);
+
+    // Invalidate the backing store buffer and repaint immediately.
+    repaintWindow();
+}
+
 void QWidgetWindow::repaintWindow()
 {
     if (!m_widget->isVisible() || !m_widget->updatesEnabled())

src/widgets/kernel/qwidgetwindow_p.h

@@ -101,9 +101,10 @@ protected:
 
 private slots:
     void updateObjectName();
-    void repaintWindow();
+    void handleScreenChange();
 
 private:
+    void repaintWindow();
     void updateGeometry();
     void updateNormalGeometry();