Background disappearence

Hi:

Consider this web page:

http://www.fc.up.pt/mp/jcsantos/quadratura.html

It has a background image. If open the file with Amaya and I press the
"Page Down" button until I get to the bottom of the page, then the
background image disappears. Is this a known bug?

I am using Amaya 9.51 under Windows XP.

Best regards,

Jose Carlos Santos