Qt-Flag: QT::CustomizeWindowHint
in class-method : QtWidgetsTweakletImpl::CreateShell(Shell::Pointer parent, int style)
in Combination with :
#ifdef APPLE
mainWindow->setUnifiedTitleAndToolBarOnMac(true);
#endif
in class: QmitkExtWorkbenchWindowAdvisor
/-------------------
comment out the flag QT::CustomizeWindowHint will solve the problem
but may produce an unpredictable behavior