Create a 2D window and then manually focus a different window on your desktop,

call SDL_PumpEvents() so the X11 driver sets its this->hidden->switch_waiting,
then set a fullscreen OpenGL window (which makes the X11 driver tear down and
create a new window instead of just resizing the existing one), poll for
events, and the newly-created window will think it needs to pop back to a
window. Fixed by resetting switch_waiting to zero if X11_CreateWindow() had
to tear down a previous window.

src/video/x11/SDL_x11video.c

@@ -665,6 +665,7 @@ static int X11_CreateWindow(_THIS, SDL_Surface *screen,
 	/* If a window is already present, destroy it and start fresh */
 	if ( SDL_Window ) {
 		X11_DestroyWindow(this, screen);
+		switch_waiting = 0; /* Prevent jump back to now-meaningless state. */
 	}
 
 	/* See if we have been given a window id */