Guillaume,
If I were you I would use a VerticalPanel to keep the content area
above the status area, and I would embed a ScrollPanel inside the top
cell of the VerticalPanel and place your scrollable content in there.
If you can get that working with some explit sizes set, you can then
look at resizing the whole composite to fill the client area. That
should keep everything where you want it, regardless of window resize
events.
I think "fixed" positioning on the web is a bit of a contriveance, and
does not do what users generally expect, not to mention the browser
incompatability problem that jhulford mentioned.
Walden