Extended menu items, when are :enable and :filter computed?

[Lennart Borgman]

I think at least for me it is impossible to understand from the 
documentation when :enable and :filter in a menu-item list is computed. 
I believe the documentation should be improved.

From my simple tests is seems like the argument to :filter is computed 
every time the menubar is accessed. It is not really what I would expect 
when this is in a submenu. Is this behaviour intentional?

The same seems to be true for the argument to :enable. Is this intentional?

What I mean is that I would expect those argument to be computed when 
they are needed, ie when the submenu is displayed.