[TD]fix crash on Dim delete

- adjust index/cache options to avoid "dirty" screen
  region errors.
- may be masking missing "prepareGeometryChange"

src/Mod/TechDraw/Gui/MDIViewPage.cpp

@@ -123,6 +123,10 @@ MDIViewPage::MDIViewPage(ViewProviderPage *pageVp, Gui::Document* doc, QWidget*
 
     setMouseTracking(true);
     m_scene = new QGraphicsScene(this);
+    m_scene->setItemIndexMethod(QGraphicsScene::NoIndex); //this prevents crash when deleting dims.
+                                                          //scene(view?) indices of dirty regions gets
+                                                          //out of sync.  missing prepareGeometryChange
+                                                          //somewhere???? QTBUG-18021???
     m_view = new QGVPage(pageVp,m_scene,this);
 
     m_toggleKeepUpdatedAction = new QAction(tr("Toggle &Keep Updated"), this);
@@ -398,9 +402,11 @@ bool MDIViewPage::attachView(App::DocumentObject *obj)
 void MDIViewPage::onDeleteObject(const App::DocumentObject& obj)
 {
     //if this page has a QView for this obj, delete it.
+    blockSelection(true);
     if (obj.isDerivedFrom(TechDraw::DrawView::getClassTypeId())) {
         (void) m_view->removeQViewByName(obj.getNameInDocument());
     }
+    blockSelection(false);
 }
 
 void MDIViewPage::onTimer() {

src/Mod/TechDraw/Gui/QGIView.cpp

@@ -627,6 +627,16 @@ MDIViewPage* QGIView::getMDIViewPage(void) const
     return MDIViewPage::getFromScene(scene());
 }
 
+//remove a child of this from scene while keeping scene indexes valid
+void QGIView::removeChild(QGIView* child)
+{
+    if ( (child != nullptr) &&
+         (child->parentItem() == this) ) {
+        prepareGeometryChange();
+        scene()->removeItem(child);
+    }
+}
+
 bool QGIView::getFrameState(void)
 {
 //    Base::Console().Message("QGIV::getFrameState() - %s\n",getViewName());

src/Mod/TechDraw/Gui/QGIView.h

@@ -122,6 +122,7 @@ public:
     static const double DefaultFontSizeInMM;
 
     MDIViewPage* getMDIViewPage(void) const;
+    virtual void removeChild(QGIView* child);
 
     // Mouse handling
     virtual void mousePressEvent(QGraphicsSceneMouseEvent *event) override;

src/Mod/TechDraw/Gui/QGIViewDimension.cpp

@@ -217,6 +217,7 @@ void QGIDatumLabel::paint(QPainter *painter, const QStyleOptionGraphicsItem *opt
 
 void QGIDatumLabel::setPosFromCenter(const double &xCenter, const double &yCenter)
 {
+    prepareGeometryChange();
     QGIViewDimension* qgivd = dynamic_cast<QGIViewDimension*>(parentItem());
     if( qgivd == nullptr ) {
         return;                  //tarfu
@@ -266,6 +267,7 @@ void QGIDatumLabel::setLabelCenter()
 
 void QGIDatumLabel::setFont(QFont f)
 {
+    prepareGeometryChange();
     m_dimText->setFont(f);
     m_unitText->setFont(f);
     QFont tFont(f);
@@ -679,6 +681,7 @@ QString QGIViewDimension::getLabelText(void)
 
 void QGIViewDimension::draw()
 {
+    prepareGeometryChange();
     if (!isVisible()) {
         return;
     }

src/Mod/TechDraw/Gui/QGVPage.cpp

@@ -137,7 +137,11 @@ QGVPage::QGVPage(ViewProviderPage *vp, QGraphicsScene* s, QWidget *parent)
     setMouseTracking(true);
     viewport()->setMouseTracking(true);
 
-    setViewportUpdateMode(QGraphicsView::SmartViewportUpdate);
+//    setViewportUpdateMode(QGraphicsView::SmartViewportUpdate);
+    setViewportUpdateMode(QGraphicsView::FullViewportUpdate); //this prevents crash when deleting dims.
+                                                          //scene(view?) indices of dirty regions gets
+                                                          //out of sync.  missing prepareGeometryChange
+                                                          //somewhere???? QTBUG-18021????
     setCacheMode(QGraphicsView::CacheBackground);
 
     Base::Reference<ParameterGrp> hGrp = App::GetApplication().GetUserParameter()
@@ -306,7 +310,8 @@ int QGVPage::removeQViewByName(const char* name)
             balloon->disconnect();
         }
         removeQViewFromScene(ourItem);
-        delete ourItem;
+        delete ourItem;              //commenting this prevents crash but means a small memory waste.
+                                     //alternate fix(?) is to change indexing/caching option in scene/view
     }
 
     return 0;
@@ -314,17 +319,13 @@ int QGVPage::removeQViewByName(const char* name)
 
 void QGVPage::removeQViewFromScene(QGIView *view)
 {
-    QGraphicsItemGroup* grp = view->group();
-    if (grp) {
-        grp->removeFromGroup(view);
-    }
-
-    if (view->parentItem()) {    //not top level
-        view->setParentItem(0);
-    }
-
-    if (view->scene()) {
-        view->scene()->removeItem(view);
+    if (view->scene() != nullptr) {
+        QGIView* qgParent = dynamic_cast<QGIView*>(view->parentItem());
+        if (qgParent != nullptr) {
+            qgParent->removeChild(view);
+        } else {
+            view->scene()->removeItem(view);
+        }
     }
 }