The edge of the sitemap (intro, what, starting, forum, chat,...,more buttons) and the forum itself.
When you scroll through the forum, sitemap stands still, y'know? You should click on any link (i.e. "Respond to this message" link) and move it UP to the edge of scrolling and fixed part of the page, still holding the mouse button (cursor must change to black circle with diagonal line in it (if you didn't change the cursors)).
