I've done some vaguely similar work  with this in the past. Looking through
some code, I get the following:

Row at top seems to be: ActiveWindow.ScrollRow
Column at left seems to be: ActiveWindow.ScrollColumn

Unfortunately, what I was doing was converting everything to pixels so I
could position popup items, so I never pursued finding the right column or
the bottom row, just the pixel extents of the window.

However, I would have used Google to find a lot of what I wanted. If you go
looking for ActiveWindow.ScrollRow and .ScrollColumn with other keywords
like "right" and "bottom", I suspect you'll find what you want.

Also, have a look in help for other attributes to ActiveWindow similar to
the two I've mentioned.