hook into window's content flux?

[Joe Corneli]

I'd like to run a function to examine the contexts that are being
displayed in my Emacs windows, whenever these change.  I'm
particularly interested in content changes related to scrolling; when
I press next-line or scroll-up, etc., the window's content changes
even though the buffer is not modified.

There are two things I think I think I'll need to know to achieve this.

1. Are there any hooks I can add to that run when the window's content
   changes?

2. Are there variables that indicate the beginning and end positions
   of the string that is being displayed on the screen?

Additional motion-dependent question, extending 1, is: are there hooks
I can run when the cursor changes position (whether or not the
window's content changes)?

These questions relate to my work on the Arxana hypertext system for
Emacs, 

http://lists.gnu.org/archive/html/gnu-emacs-sources/2006-03/msg00017.html

and particularly to the problem of contextual display of annotations.