Revert "workaround for a really strange behavior on x11 (cf. comments).."

It still happens if a virtual machine is opened (window manager issue/conflicts ?) but I can't reproduce if not. And it provokes a regression with multiple screens

This reverts commit 5c517525fc.

src/core/UBDisplayManager.cpp

@@ -231,29 +231,8 @@ void UBDisplayManager::positionScreens()
     {
         mControlWidget->hide();
         mControlWidget->setGeometry(mDesktop->availableGeometry(mControlWidget));
-
-#ifdef Q_OS_LINUX
-        /*
-         * in Gnome (X11), a very strange behavior randomly happens if this call is done at app start (the transparent drawing view of the desktop mode
-         * becomes somewhat a screenshot of the gnome desktop, so no interaction is possible through the view, and a second "screenshot" (or a second "thread" of the window ? ...) of the desktpop appears ...
-         *
-         * It seems to be already assumed by Qt though, so not sure if a qtbug should be created..
-         *
-         * A known regression is that starting OpenBoard in DesktopMode directly may not handle perfectly until a screen mode change occurs (going to BoardMode and go back to Desktop Mode)
-         * (left palette closed by default and pen size way too large...) but it is less blocking than the other issue.
-         *
-         * Maybe To be able to start in DesktopMode should be reconsidered as it seems to work only on Windows at this time (or available only on Windows)
-         *
-         * https://doc.qt.io/qt-5/qwidget.html#showFullScreen
-        */
-        if (qgetenv("XDG_SESSION_TYPE") != "x11")
-        {
         UBPlatformUtils::showFullScreen(mControlWidget);
     }
-#else
-        UBPlatformUtils::showFullScreen(mControlWidget);
-#endif
-    }
 
     if (mDisplayWidget && mDisplayScreenIndex > -1)
     {