When a window is activated (e.g. when you click on a non-front window), Smile sends a deactivated event to the object's script. of the former front window and a activated event to the object's script of the new front window. You handle activated (or deactivated) with a handler such as:

on activated the_window
    -- do whatever suitable
    continue activated the_window
end activated

the_window contains a reference to the window being activated, the owner of the script.