inconsistency btw viewport and image navigation window

[mromanello]

Bug reported by John Bradley (per email on 12/03/2012 )

It is Firefox 10.0.2 on a Vista Windows system. As I recall, both navigation buttons and dragging could produce the same result.
[...] the little box in the little image doesn't seem to always actually track the full area correctly.
See attached image where the actually image area shows the area of the picture that goes below the bottom of th epage of
music, but that the little box in the navigation image suggests that it is significantly above the bottom of the image.