Fix tooltip positioning

The previous fix worked only properly when the QQuickWidget was located in
the top-left corner of the window. With bigger offsets the tooltips were off.

Task-number: QTBUG-39878
Change-Id: Ia25194175aa9f9daa9c7132fc776d4b2fa874c26
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

src/controls/Private/qquicktooltip.cpp

@@ -66,9 +66,11 @@ void QQuickTooltip::showText(QQuickItem *item, const QPointF &pos, const QString
     if (QGuiApplicationPrivate::platformIntegration()->
             hasCapability(QPlatformIntegration::MultipleWindows) &&
         QCoreApplication::instance()->inherits("QApplication")) {
-        QWindow *renderWindow = QQuickRenderControl::renderWindowFor(item->window());
+        QPoint quickWidgetOffsetInTlw;
+        QWindow *renderWindow = QQuickRenderControl::renderWindowFor(item->window(), &quickWidgetOffsetInTlw);
         QWindow *window = renderWindow ? renderWindow : item->window();
-        QPoint mappedPos = window->mapToGlobal(item->mapToScene(pos).toPoint());
+        const QPoint offsetInQuickWidget = item->mapToScene(pos).toPoint();
+        const QPoint mappedPos = window->mapToGlobal(offsetInQuickWidget + quickWidgetOffsetInTlw);
         QToolTip::showText(mappedPos, str);
     }
 #else