workaround for broken tree indicators with Qt 5.6 under Windows

src/Gui/DlgGeneralImp.cpp

@@ -195,6 +195,22 @@ void DlgGeneralImp::saveSettings()
             qApp->sendEvent(getMainWindow(), &e);
             mdi->setBackground(QBrush(QColor(160,160,160)));
         }
+
+#if QT_VERSION == 0x050600 && defined(Q_OS_WIN32)
+        // Under Windows the tree indicator branch gets corrupted after a while.
+        // For more details see also https://bugreports.qt.io/browse/QTBUG-52230
+        // and https://codereview.qt-project.org/#/c/154357/2//ALL,unified
+        // A workaround for Qt 5.6.0 is to set a minimal style sheet.
+        QString qss = QString::fromLatin1(
+               "QTreeView::branch:closed:has-children  {\n"
+               "    image: url(:/icons/style/windows_branch_closed.png);\n"
+               "}\n"
+               "\n"
+               "QTreeView::branch:open:has-children  {\n"
+               "    image: url(:/icons/style/windows_branch_open.png);\n"
+               "}\n");
+        qApp->setStyleSheet(qss);
+#endif
     }
 
     if (mdi->style())