Override virtual method in private class instead of downcasting

Move QGraphicsView specific code into reimplementation of now virtual
(and const) canStartScrollingAt.

Remove unhelpful comment.

Change-Id: Ib4799e48ac4f85748c77c52d29511a0490303676
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/widgets/graphicsview/qgraphicsview.cpp

@@ -613,6 +613,26 @@ void QGraphicsViewPrivate::updateScroll()
     dirtyScroll = false;
 }
 
+/*!
+    \internal
+
+    * don't start scrolling when a drag mode has been set
+    * don't start scrolling on a movable item
+*/
+bool QGraphicsViewPrivate::canStartScrollingAt(const QPoint &startPos) const
+{
+    Q_Q(const QGraphicsView);
+    if (q->dragMode() != QGraphicsView::NoDrag)
+        return false;
+
+    const QGraphicsItem *childItem = q->itemAt(startPos);
+
+    if (childItem && (childItem->flags() & QGraphicsItem::ItemIsMovable))
+        return false;
+
+    return QAbstractScrollAreaPrivate::canStartScrollingAt(startPos);
+}
+
 /*!
     \internal
 */

src/widgets/graphicsview/qgraphicsview_p.h

@@ -117,6 +117,7 @@ public:
     QTransform matrix;
     qint64 scrollX, scrollY;
     void updateScroll();
+    bool canStartScrollingAt(const QPoint &startPos) const override;
 
     qreal leftIndent;
     qreal topIndent;

src/widgets/widgets/qabstractscrollarea.cpp

@@ -1361,28 +1361,13 @@ void QAbstractScrollArea::scrollContentsBy(int, int)
     viewport()->update();
 }
 
-bool QAbstractScrollAreaPrivate::canStartScrollingAt( const QPoint &startPos )
+bool QAbstractScrollAreaPrivate::canStartScrollingAt(const QPoint &startPos) const
 {
-    Q_Q(QAbstractScrollArea);
-
-#if QT_CONFIG(graphicsview)
-    // don't start scrolling when a drag mode has been set.
-    // don't start scrolling on a movable item.
-    if (QGraphicsView *view = qobject_cast<QGraphicsView *>(q)) {
-        if (view->dragMode() != QGraphicsView::NoDrag)
-            return false;
-
-        QGraphicsItem *childItem = view->itemAt(startPos);
-
-        if (childItem && (childItem->flags() & QGraphicsItem::ItemIsMovable))
-            return false;
-    }
-#endif
+    Q_Q(const QAbstractScrollArea);
 
     // don't start scrolling on a QAbstractSlider
-    if (qobject_cast<QAbstractSlider *>(q->viewport()->childAt(startPos))) {
+    if (qobject_cast<QAbstractSlider *>(q->viewport()->childAt(startPos)))
         return false;
-    }
 
     return true;
 }

src/widgets/widgets/qabstractscrollarea_p.h

@@ -94,9 +94,8 @@ public:
     void init();
     void layoutChildren();
     void layoutChildren_helper(bool *needHorizontalScrollbar, bool *needVerticalScrollbar);
-    // ### Fix for 4.4, talk to Bjoern E or Girish.
     virtual void scrollBarPolicyChanged(Qt::Orientation, Qt::ScrollBarPolicy) {}
-    bool canStartScrollingAt( const QPoint &startPos );
+    virtual bool canStartScrollingAt( const QPoint &startPos ) const;
 
     void flashScrollBars();
     void setScrollBarTransient(QScrollBar *scrollBar, bool transient);