Invoked when window receives a VisibilityUnobscured event.\n\ The hook procedures are invoked with two arguments: the window object\n\ of the window that is now fully obscured, resulting-from-viewport-move? (a boolean).\n\ Beware that this event happens more often than you might expect and an action procedure\n\ attached here should be very careful about manipulating windows in a way\n\ that might cause more Visibility events. See also window-visibility.