Introduce events for Window device pixel ratio changes

There is a mix between screen device pixel ratio. Currently we store the
property on a per-window basis, but the change notifications are still
on a per screen basis which can fall apart on edge cases.

On wayland we are getting per window DPR changes without as useful
screen change events so it's important to fix. It also has potential to
clean up the Windows backend in the future where the backend is
currently papering over the two concepts.

This patch introduces two new events:

A QWindowSystemInterface to trigger a window DPR change
independently of a screen change.

An event to notify windows the new DPR rather than needing to track
signals on the screen. This happens either when the window dpr changes
or implicitly through a screen change. This can deprecate an existing
event ScreenChangeInternal so the value is reused and renamed for
clarity.

Change-Id: I637a07fd4520ba3184ccc2c987c29d8d23a65ad3
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/kernel/qcoreevent.cpp

@@ -85,6 +85,7 @@ Q_TRACE_POINT(qtcore, QEvent_dtor, QEvent *event, QEvent::Type type);
     \value ContextMenu                      Context popup menu (QContextMenuEvent).
     \value CursorChange                     The widget's cursor has changed.
     \value DeferredDelete                   The object will be deleted after it has cleaned up (QDeferredDeleteEvent)
+    \value DevicePixelRatioChange           The devicePixelRatio has changed for this widget's or window's underlying backing store
     \value DragEnter                        The cursor enters a widget during a drag and drop operation (QDragEnterEvent).
     \value DragLeave                        The cursor leaves a widget during a drag and drop operation (QDragLeaveEvent).
     \value DragMove                         A drag and drop operation is in progress (QDragMoveEvent).

src/corelib/kernel/qcoreevent.h

@@ -284,6 +284,8 @@ public:
         // GraphicsSceneLeave = 220,
         WindowAboutToChangeInternal = 221,      // internal for QQuickWidget and texture-based widgets
 
+        DevicePixelRatioChange = 222,
+
         // 512 reserved for Qt Jambi's MetaCall event
         // 513 reserved for Qt Jambi's DeleteOnMainThread event
 

src/gui/kernel/qguiapplication.cpp

@@ -2052,6 +2052,9 @@ void Q_TRACE_INSTRUMENT(qtgui) QGuiApplicationPrivate::processWindowSystemEvent(
     case QWindowSystemInterfacePrivate::WindowScreenChanged:
         QGuiApplicationPrivate::processWindowScreenChangedEvent(static_cast<QWindowSystemInterfacePrivate::WindowScreenChangedEvent *>(e));
         break;
+    case QWindowSystemInterfacePrivate::WindowDevicePixelRatioChanged:
+        QGuiApplicationPrivate::processWindowDevicePixelRatioChangedEvent(static_cast<QWindowSystemInterfacePrivate::WindowDevicePixelRatioChangedEvent *>(e));
+        break;
     case QWindowSystemInterfacePrivate::SafeAreaMarginsChanged:
         QGuiApplicationPrivate::processSafeAreaMarginsChangedEvent(static_cast<QWindowSystemInterfacePrivate::SafeAreaMarginsChangedEvent *>(e));
         break;
@@ -2555,6 +2558,9 @@ void QGuiApplicationPrivate::processWindowScreenChangedEvent(QWindowSystemInterf
     if (QWindow *window  = wse->window.data()) {
         if (window->screen() == wse->screen.data())
             return;
+
+        const qreal oldDevicePixelRatio = window->screen() ? window->screen()->devicePixelRatio() : 1.0;
+
         if (QWindow *topLevelWindow = window->d_func()->topLevelWindow(QWindow::ExcludeTransients)) {
             if (QScreen *screen = wse->screen.data())
                 topLevelWindow->d_func()->setTopLevelScreen(screen, false /* recreate */);
@@ -2571,9 +2577,24 @@ void QGuiApplicationPrivate::processWindowScreenChangedEvent(QWindowSystemInterf
             processGeometryChangeEvent(&gce);
         }
 #endif
+
+        const qreal newDevicePixelRatio = window->screen() ? window->screen()->devicePixelRatio() : 1.0;
+        if (!qFuzzyCompare(oldDevicePixelRatio, newDevicePixelRatio)) {
+            QEvent dprChangeEvent(QEvent::DevicePixelRatioChange);
+            QGuiApplication::sendSpontaneousEvent(window, &dprChangeEvent);
+        }
     }
 }
 
+void QGuiApplicationPrivate::processWindowDevicePixelRatioChangedEvent(QWindowSystemInterfacePrivate::WindowDevicePixelRatioChangedEvent *wde)
+{
+    if (wde->window.isNull())
+        return;
+
+    QEvent dprChangeEvent(QEvent::DevicePixelRatioChange);
+    QGuiApplication::sendSpontaneousEvent(wde->window, &dprChangeEvent);
+}
+
 void QGuiApplicationPrivate::processSafeAreaMarginsChangedEvent(QWindowSystemInterfacePrivate::SafeAreaMarginsChangedEvent *wse)
 {
     if (wse->window.isNull())

src/gui/kernel/qguiapplication_p.h

@@ -116,6 +116,7 @@ public:
     static void processActivatedEvent(QWindowSystemInterfacePrivate::ActivatedWindowEvent *e);
     static void processWindowStateChangedEvent(QWindowSystemInterfacePrivate::WindowStateChangedEvent *e);
     static void processWindowScreenChangedEvent(QWindowSystemInterfacePrivate::WindowScreenChangedEvent *e);
+    static void processWindowDevicePixelRatioChangedEvent(QWindowSystemInterfacePrivate::WindowDevicePixelRatioChangedEvent *e);
 
     static void processSafeAreaMarginsChangedEvent(QWindowSystemInterfacePrivate::SafeAreaMarginsChangedEvent *e);
 

src/gui/kernel/qwindow.cpp

@@ -505,6 +505,8 @@ void QWindowPrivate::create(bool recursive, WId nativeHandle)
     // the platformWindow, if there was one, is now gone, so make this flag reflect reality now
     updateRequestPending = false;
 
+    const qreal currentDevicePixelRatio = q->devicePixelRatio();
+
     if (q->parent())
         q->parent()->create();
 
@@ -550,6 +552,11 @@ void QWindowPrivate::create(bool recursive, WId nativeHandle)
     QPlatformSurfaceEvent e(QPlatformSurfaceEvent::SurfaceCreated);
     QGuiApplication::sendEvent(q, &e);
 
+    if (!qFuzzyCompare(currentDevicePixelRatio, q->devicePixelRatio())) {
+        QEvent dprChangeEvent(QEvent::DevicePixelRatioChange);
+        QGuiApplication::sendEvent(q, &dprChangeEvent);
+    }
+
     if (needsUpdate)
         q->requestUpdate();
 }

src/gui/kernel/qwindowsysteminterface.cpp

@@ -259,6 +259,12 @@ QT_DEFINE_QPA_EVENT_HANDLER(void, handleWindowScreenChanged, QWindow *window, QS
     handleWindowSystemEvent<QWindowSystemInterfacePrivate::WindowScreenChangedEvent, Delivery>(window, screen);
 }
 
+QT_DEFINE_QPA_EVENT_HANDLER(void, handleWindowDevicePixelRatioChanged, QWindow *window)
+{
+    handleWindowSystemEvent<QWindowSystemInterfacePrivate::WindowDevicePixelRatioChangedEvent, Delivery>(window);
+}
+
+
 QT_DEFINE_QPA_EVENT_HANDLER(void, handleSafeAreaMarginsChanged, QWindow *window)
 {
     handleWindowSystemEvent<QWindowSystemInterfacePrivate::SafeAreaMarginsChangedEvent, Delivery>(window);

src/gui/kernel/qwindowsysteminterface.h

@@ -190,6 +190,9 @@ public:
     template<typename Delivery = QWindowSystemInterface::DefaultDelivery>
     static void handleWindowScreenChanged(QWindow *window, QScreen *newScreen);
 
+    template<typename Delivery = QWindowSystemInterface::DefaultDelivery>
+    static void handleWindowDevicePixelRatioChanged(QWindow *window);
+
     template<typename Delivery = QWindowSystemInterface::DefaultDelivery>
     static void handleSafeAreaMarginsChanged(QWindow *window);
 

src/gui/kernel/qwindowsysteminterface_p.h

@@ -68,7 +68,8 @@ public:
         WindowScreenChanged = 0x21,
         SafeAreaMarginsChanged = 0x22,
         ApplicationTermination = 0x23,
-        Paint = 0x24
+        Paint = 0x24,
+        WindowDevicePixelRatioChanged = 0x25,
     };
 
     class WindowSystemEvent {
@@ -154,6 +155,15 @@ public:
         QPointer<QScreen> screen;
     };
 
+    class WindowDevicePixelRatioChangedEvent : public WindowSystemEvent {
+    public:
+        WindowDevicePixelRatioChangedEvent(QWindow *w)
+            : WindowSystemEvent(WindowDevicePixelRatioChanged), window(w)
+        { }
+
+        QPointer<QWindow> window;
+    };
+
     class SafeAreaMarginsChangedEvent : public WindowSystemEvent {
     public:
         SafeAreaMarginsChangedEvent(QWindow *w)

src/openglwidgets/qopenglwidget.cpp

@@ -1690,7 +1690,7 @@ bool QOpenGLWidget::event(QEvent *e)
             }
         }
         break;
-    case QEvent::ScreenChangeInternal:
+    case QEvent::DevicePixelRatioChange:
         if (d->initialized && d->paintDevice->devicePixelRatio() != devicePixelRatio())
             d->recreateFbos();
         break;

src/widgets/kernel/qwidget.cpp

@@ -9318,6 +9318,8 @@ bool QWidget::event(QEvent *event)
             const QWindow *win = te->window;
             d->setWinId((win && win->handle()) ? win->handle()->winId() : 0);
         }
+        break;
+    case QEvent::DevicePixelRatioChange:
         if (d->data.fnt.d->dpi != logicalDpiY())
             d->updateFont(d->data.fnt);
         d->renderToTextureReallyDirty = 1;

src/widgets/kernel/qwidgetwindow.cpp

@@ -329,6 +329,10 @@ bool QWidgetWindow::event(QEvent *event)
         m_widget->repaint();
         return true;
 
+    case QEvent::DevicePixelRatioChange:
+        handleDevicePixelRatioChange();
+        break;
+
     default:
         break;
     }
@@ -695,22 +699,32 @@ void QWidgetWindow::updateMargins()
     m_widget->data->fstrut_dirty = false;
 }
 
-static void sendScreenChangeRecursively(QWidget *widget)
+static void sendChangeRecursively(QWidget *widget, QEvent::Type type)
 {
-    QEvent e(QEvent::ScreenChangeInternal);
+    QEvent e(type);
     QCoreApplication::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);
+            sendChangeRecursively(w, type);
     }
 }
 
 void QWidgetWindow::handleScreenChange()
 {
     // Send an event recursively to the widget and its children.
-    sendScreenChangeRecursively(m_widget);
+    sendChangeRecursively(m_widget, QEvent::ScreenChangeInternal);
+
+    // Invalidate the backing store buffer and repaint immediately.
+    if (screen())
+        repaintWindow();
+}
+
+void QWidgetWindow::handleDevicePixelRatioChange()
+{
+    // Send an event recursively to the widget and its children.
+    sendChangeRecursively(m_widget, QEvent::DevicePixelRatioChange);
 
     // Invalidate the backing store buffer and repaint immediately.
     if (screen())

src/widgets/kernel/qwidgetwindow_p.h

@@ -81,9 +81,10 @@ protected:
 
 private slots:
     void updateObjectName();
-    void handleScreenChange();
 
 private:
+    void handleScreenChange();
+    void handleDevicePixelRatioChange();
     void repaintWindow();
     bool updateSize();
     void updateMargins();

src/widgets/widgets/qdockwidget.cpp

@@ -119,7 +119,7 @@ bool QDockWidgetTitleButton::event(QEvent *event)
 {
     switch (event->type()) {
     case QEvent::StyleChange:
-    case QEvent::ScreenChangeInternal:
+    case QEvent::DevicePixelRatioChange:
         m_iconSize = -1;
         break;
     default: