There's nothing in the GSX code that either knows or check or handles undocking the menu, let alone knowing if it has been docked alongside the main MSFS window ( assuming MSFS is running in a window ) but still on the main monitor, or has docked on a secondary monitor.
Whatever behavior you see, like the menu going black for example, is nothing we coded explicitly, it's done automatically by the sim.
Have you tried undocking but keeping the menu on the main screen ? I'm trying to understand is the issue is just when the undocked menu is located on another monitor.