If the video system has been initialized, only use that message box system.
Don't pass a window from one video driver to the messagebox function of another video driver.  This makes bad things happen. :)
Make sure a window is valid for a subsystem before using it in a messagebox
Removed obsolete assertion code
Fixed code example for SDL_GetWindowWMInfo()
Added testing of messagebox with a parent window
Fixed bug 1970 - Cocoa message boxes ignore parent window requests

Ryan C. Gordon

Cocoa_ShowMessageBox() ignores the "window" field of SDL_MessageBoxData, which means you can't assign a parent window to a message box. This is particularly egregious on Mac OS X, because it'll actually make the NSAlert visually part of the parent window instead of just concerning itself with window focus.