No menu at the top of the page when scrolling in IE6 while in IE7 & FF3.5.2 it's held in place at the top

Steps to follow to reproduce:

• create a new page and go to edit mode
• write some text
• click "Save & Continue"
• in view mode scroll down

Result: You will see no menu at the top of the page in IE6, while in IE7 and FF3.5.2 it's held in place at the top (see attached images).