[css-device-adapt] How should initial viewport contribute to layout?

Hi,

Actually I have 2 related questions:
1. Should fixed-position blocks layout according to the 'initial viewport',
'actual viewport' or something between them?
2. For a page with unconstrained viewport height, how should the height of
the 'actual viewport' be adjusted when there is temporary height change of
the 'initial viewport', e.g. when the on-screen-keyboard shows?

The answers to the above questions seem to have already been covered by
existing specs, but after we examine the current behavior of different
mobile browsers we found they have tweaked the existing specs to achieve
more user-friendship and/or better performance:

1. All browsers seems not to layout top-level fixed-position blocks
according to the initial-containing-block (i.e. 'actual viewport' in
css-device-adapt), but somehow according to the visible viewport (i.e.
'initial viewport' in css-device-adapt). Mobile Safari's behavior is more
complex that seems to use a bigger virtual viewport to contain the
fixed-position elements if the visible viewport is too small.

2. For a page with unconstrained viewport height, the browsers behave
differently about whether to adjust the actual viewport height when
visibility of the top- and/or bottom- controls and the on-screen keyboard
changes.

https://docs.google.com/a/chromium.org/document/d/1ukfEZGJQq-7VD_Tv9m_cYD4BoR6YZots5_KddtvYZJs/edit#summarizes
the current behaviors of the browsers.

I think it'd be better if the spec address the issues that caused the
browsers to tweak the spec, to avoid inconsistency and confusion.

Thanks,
Xianzhu