OnWindowVisibilityChanged() is sent in two distinct cases:
. when the visibility of the window the observer was added to changes.
. when the visibility of an ancestor of (or descendant) changes.

It's extremely easy to miss the latter and leads to subtle bugs. To make this more obvious we should introduce explicit functions for the latter case, problem:

OnWindowVisibilityOfAncestorChanged();
OnWindowVisibilityOfDescendantChanged();

Or pass in a parameter to OnWindowVisibilityChanged() indicating the relation. Either one makes it more obvious when the function is called.