workaround to broken StaysOnTop+TransluscentBackground behavior on OSX 10.13+ (cf. https://bugreports.qt.io/browse/QTBUG-81456). Not satisfying at all but only thing I found acceptable for the moment...

src/desktop/UBDesktopAnnotationController.cpp

@@ -84,8 +84,10 @@ UBDesktopAnnotationController::UBDesktopAnnotationController(QObject *parent, UB
     mTransparentDrawingView->setAttribute(Qt::WA_TranslucentBackground, true);
 #ifdef Q_OS_OSX
     mTransparentDrawingView->setAttribute(Qt::WA_MacNoShadow, true);
-#endif
+    mTransparentDrawingView->setWindowFlags(Qt::FramelessWindowHint | Qt::WindowStaysOnBottomHint | Qt::Window | Qt::NoDropShadowWindowHint | Qt::X11BypassWindowManagerHint);
+#else
     mTransparentDrawingView->setWindowFlags(Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint | Qt::Window | Qt::NoDropShadowWindowHint | Qt::X11BypassWindowManagerHint);
+#endif
     mTransparentDrawingView->setCacheMode(QGraphicsView::CacheNone);
     mTransparentDrawingView->resize(UBApplication::desktop()->width(), UBApplication::desktop()->height());