I think it is something like a temporary zoom - fist double click zooms in, the second out. There's normal zoom mapped to the +/- keys. No context menu as far as i can see.

The frameset behavior is even stranger, look here:
http://www.dartmouth.edu/~milton/rea...ts/index.shtml

When you drag down it only scrolls to where the first frame ends, not the longer one.