Is your window you are working on maximized? I have had that problem when the window has opened in less than full size.
If you turn off the rulers in View, User Interface, then this problem will go away. It is logged as SPR# 964182. If you want to "vote" for the SPR and be notified when it is fixed, please go to the Knowledge Base in the customer portal, search for 964182 and choose the option on the left to be notified when it is fixed.
Thank you, that worked!
(I've never even looked at the ruler before)