Fix QGraphicsView's abuse of QEvent's d-pointer in Leave event handling

QGraphicsSceneEvent carries a widget pointer, and we can add a new
event
type. Assert that QGraphicsScene doesn't get Leave events from
elsewhere.

This was the only "usage" of QEvent's d-pointer in Qt, so with this gone
we can reduce the size of QEvent by 8 bytes.

Change-Id: I0513daf742de9084d0469d55b7a7f41b667e8081
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/kernel/qcoreevent.cpp

@@ -153,6 +153,7 @@ QT_BEGIN_NAMESPACE
     \value GraphicsSceneMove                Widget was moved (QGraphicsSceneMoveEvent).
     \value GraphicsSceneResize              Widget was resized (QGraphicsSceneResizeEvent).
     \value GraphicsSceneWheel               Mouse wheel rolled in a graphics scene (QGraphicsSceneWheelEvent).
+    \value GraphicsSceneLeave               The cursor leaves a graphics scene (QGraphicsSceneWheelEvent).
     \value Hide                             Widget was hidden (QHideEvent).
     \value HideToParent                     A child widget has been hidden.
     \value HoverEnter                       The mouse cursor enters a hover widget (QHoverEvent).

src/corelib/kernel/qcoreevent.h

@@ -205,6 +205,7 @@ public:
         GraphicsSceneDragLeave = 166,
         GraphicsSceneDrop = 167,
         GraphicsSceneWheel = 168,
+        GraphicsSceneLeave = 220,
 
         KeyboardLayoutChange = 169,             // keyboard layout changed
 

src/widgets/graphicsview/qgraphicsscene.cpp

@@ -3371,9 +3371,15 @@ bool QGraphicsScene::event(QEvent *event)
         break;
     }
     case QEvent::Leave:
-        // hackieshly unpacking the viewport pointer from the leave event.
+        Q_ASSERT_X(false, "QGraphicsScene::event",
-        d->leaveScene(reinterpret_cast<QWidget *>(event->d));
+                   "QGraphicsScene must not receive QEvent::Leave, use GraphicsSceneLeave");
         break;
+    case QEvent::GraphicsSceneLeave:
+    {
+        auto *leaveEvent = static_cast<QGraphicsSceneEvent*>(event);
+        d->leaveScene(leaveEvent->widget());
+        break;
+    }
     case QEvent::GraphicsSceneHelp:
         helpEvent(static_cast<QGraphicsSceneHelpEvent *>(event));
         break;

src/widgets/graphicsview/qgraphicsview.cpp

@@ -2871,11 +2871,10 @@ bool QGraphicsView::viewportEvent(QEvent *event)
                 d->scene->d_func()->removePopup(d->scene->d_func()->popupWidgets.constFirst());
         }
         d->useLastMouseEvent = false;
-        // a hack to pass a viewport pointer to the scene inside the leave event
+        QGraphicsSceneEvent leaveEvent(QEvent::GraphicsSceneLeave);
-        Q_ASSERT(event->d == nullptr);
+        leaveEvent.setWidget(viewport());
-        QScopedValueRollback<QEventPrivate *> rb(event->d);
+        QCoreApplication::sendEvent(d->scene, &leaveEvent);
-        event->d = reinterpret_cast<QEventPrivate *>(viewport());
+        event->setAccepted(leaveEvent.isAccepted());
-        QCoreApplication::sendEvent(d->scene, event);
         break;
     }
 #if QT_CONFIG(tooltip)