Hello David
I had experienced similar problem with one of my users here. Other than things which you have already tried(Reinstalling SAP GUI), we did a few more things to (finally) resolve the issue on her system
1. Adjusting the screen resolution. --This did not help. hence tried#2 below
2. Re-installing the display driver. -- This helped resolved the issue.
Please refer these thread for additional details.
Parameter required for Scroll up and down in APO Planning Books
Also recently this blog was posted on SCN which tells about font setting to be rest to 11(default).
Missing scroll bars in Interactive Demand Planning
Hope this will help you.
Thank you
Satish Waghmare