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.

/*   SDLMain.m - main entry point for our Cocoa-ized SDL app
       Initial Version: Darrell Walisser <dwaliss1@purdue.edu>
       Non-NIB-Code & other changes: Max Horn <max@quendi.de>

    Feel free to customize this file to suit your needs
*/

#import <Cocoa/Cocoa.h>

@interface SDLMain : NSObject
@end