workaround for broken tree indicators with Qt 5.6 under Windows

src/Gui/Application.cpp

@@ -1780,6 +1780,23 @@ void Application::runApplication(void)
             qApp->sendEvent(&mw, &e);
         }
     }
+#if QT_VERSION == 0x050600 && defined(Q_OS_WIN32)
+    else {
+        // 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
 
     //initialize spaceball.
     mainApp.initSpaceball(&mw);

src/Gui/Icons/resource.qrc

@@ -40,6 +40,8 @@
         <file>ClassBrowser/type_class.svg</file>
         <file>ClassBrowser/type_enum.svg</file>
         <file>ClassBrowser/type_module.svg</file>
+        <file>style/windows_branch_closed.png</file>
+        <file>style/windows_branch_open.png</file>
         <file>Std_ViewScreenShot.svg</file>
         <file>bound-expression.svg</file>
         <file>bound-expression-unset.svg</file>