navigating cleanly between main- and sub-stacks

[Richmond Mathewson]

Wow, another naive question from Richmond . . .  :)

What does "cleanly" in "navigating cleanly" mean?

I assume (possibly wrongly) that you mean that the
end-user is unaware s/he is actually changing from
the main stack to a substack.

If that is so important why do you bother to have a substack at
all (memory overheads? multimedia storage?) when you
could just navigate to another part of the main stack?

Personally, when I use substacks it is because I want the
end-user to experience 2 'windows' or a 'window' and a
palette on-screen.

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The natural response to "navigating cleanly" is to
ask what "navigating dirtily" is; and presumably the
answer should come from Steve Jobs who doesn't
seem very keen on it given the 'thing' with the
'fleshly' iPhone apps . . .  :)