Since many different event structures include windowID it should be placed near the beginning of the structure (preferably right after type) so it's position is the same between different events.

This is to avoid code like this:
if (event.type == SDL_WINDOWEVENT)
    win = event.window.windowID;
else if ((SDL_EVENTMASK(event.type) & SDL_KEYEVENTMASK) != 0)
    win = event.key.windowID;
else if (event.type == SDL_TEXTINPUT)
    win = event.text.windowID;
else if (event.type == SDL_MOUSEMOTION)
    win = event.motion.windowID;
else if ((SDL_EVENTMASK(event.type) & (SDL_MOUBUTTONDOWNMASK | SDL_MOUBUTTONUPMASK)) != 0)
    win = event.button.windowID;
else if (event.type == SDL_MOUSEWHEEL)
    win = event.wheel.windowID;
...

in favor of:
win = event.window.windowID;