Window coordinates are in the global space and windows are not tied to a particular display.

Also added Ctrl-Enter keybinding to the test code to toggle fullscreen mode for testing.

src/video/x11/SDL_x11opengl.c

@@ -380,7 +380,7 @@ X11_GL_CreateContext(_THIS, SDL_Window * window)
     SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
     Display *display = data->videodata->display;
     int screen =
-        ((SDL_DisplayData *) window->display->driverdata)->screen;
+        ((SDL_DisplayData *) SDL_GetDisplayForWindow(window)->driverdata)->screen;
     XWindowAttributes xattr;
     XVisualInfo v, *vinfo;
     int n;