gain 0 if the event is a loss or 1 if it is a gain.
SDL_!ActiveEvent? is a member of the SDL_Event union and is used when an event of type SDL_ACTIVEEVENT is reported.
When the mouse leaves or enters the window area a SDL_APPMOUSEFOCUS type activation event occurs, if the mouse entered the window then gain will be 1, otherwise gain will be 0. A SDL_APPINPUTFOCUS type activation event occurs when the application loses or gains keyboard focus. This usually occurs when another application is made active. Finally, a SDL_APPACTIVE type event occurs when the application is either minimised/iconified (gain=0) or restored.