Get the window size for the window receiving the mouse motion

This is the mouse focus except in the case where relative motion is enabled and the mouse is over a window floating on top of the application window (e.g. the taskbar)

src/events/SDL_mouse.c

@@ -412,8 +412,7 @@ SDL_PrivateSendMouseMotion(SDL_Window * window, SDL_MouseID mouseID, int relativ
     if (window && ((window->flags & SDL_WINDOW_MOUSE_CAPTURE) == 0)) {
         int x_max = 0, y_max = 0;
 
-        /* !!! FIXME: shouldn't this be (window) instead of (mouse->focus)? */
-        SDL_GetWindowSize(mouse->focus, &x_max, &y_max);
+        SDL_GetWindowSize(window, &x_max, &y_max);
         --x_max;
         --y_max;