I would describe the behaviour as follows: if I do not have to scroll down to diaply the page content, the tool tip display ("footnote hover text") appears close to the footnote indicator over which you hover. The further I scroll down the page, the farer the tooltip display with the footnote content is displaced against the top of the page, i.e. even if the footnote indicator over which I hover is at the bottom of the displayed page, the tooltip text appears more and more to the top of the displayed page at a distance to the footnote indicator. Finally, if you scroll down enough (like on the Syntax page), the tool tip display ("footnote hover text") "disappears through the top" of the displayed page, I guess that the position where the system tries to display the tooltip is above the actual display area.
The included wiki-text file should demonstrate the beviour. I am using IE8 as a brouwser and "2009-12-25 Lemming" on a Synology DS710+.