on-activate in top-level-window<%>
Called when a window is activated or deactivated. A top-level window is activated when the keyboard focus moves from outside the window to the window or one of its children. It is deactivated when the focus moves back out of the window. Under Mac OS X, a child of a floating frames can have the focus instead of a child of the active non-floating frame; in other words, floating frames act as an extension of the active non-frame for keyboard focus.
The method's argument is #t when the window is activated,
#f when it is deactivated.