do not take snapshot if region is not visible

src/dtkWidgets/dtkWidgetsWorkspaceStackBar.cpp

@@ -113,6 +113,8 @@ void dtkWidgetsWorkspaceStackBarItem::setSource(QWidget *source)
     timer->setSingleShot(false);
 
     connect(timer, &QTimer::timeout, [=] (void) {
+        if (this->visibleRegion().isNull())
+            return;
         QPixmap pixmap(this->source->size());
         this->source->render(&pixmap);
         this->overview->setPixmap(pixmap.scaledToWidth(280));