When a window containing a QOpenGLWidget in fullscreen mode switches from one monitor to another (due to a disconnect) then popups on the widget no longer show up. This happens even if the setHasBorderInFullScreen() is turned on. The popup shows up fine on the original monitor before it is disconnected. Using QOpenGLWindow in a container works fine.
With the example, move the window to the monitor that will be disconnected. Press F then right click to see the popup shows up fine. Disconnect the monitor and then right click on the window on the other monitor to see that the popup is not visible.