If an exclusive fullscreen window is moved between displays, SDL_UpdateFullscrrenMode can kick the window out of fullscreen if the display onto which it was moved doesn't have a matching video mode. Check the return code and clear the fullscreen flag and skip the resize if the window is no longer fullscreen.
Simple DirectMedia Layer is a cross-platform development library designed
to provide low level access to audio, keyboard, mouse, joystick, and graphics
hardware via OpenGL and Direct3D. It is used by video playback software,
emulators, and popular games including Valve's award winning catalog
and many Humble Bundle games.
More extensive documentation is available in the docs directory, starting
with README.md