The Cure
Change the workspace layout to something else and then change it back. The workspace layout can be accesed via windows > workspace layout or via the drop down list normally at the top (to the right) of the application window. I normally use the designer layout so when I have this problem I change to anything else, then back to designer and the problem is gone.... at least for a while.
Be nice to offer a permanent solution.... but just like dear old Bono - I just haven't found what I'm looking for,