The compositor can toggle the fullscreen state (via a hotkey or otherwise), so the internal SDL state must be updated accordingly when it does.
When toggling fullscreen via the compositor, SDL will attempt to use the last fullscreen flag explicitly set. If no flag was previously set, SDL_WINDOW_FULLSCREEN will be used if a window video mode was set, otherwise SDL_WINDOW_FULLSCREEN_DESKTOP will be used. If the previous flag was SDL_WINDOW_FULLSCREEN and the window video mode was cleared, it will revert to SDL_WINDOW_FULLSCREEN_DESKTOP.
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