Thank you for your post.
I am unable to replicate this on 10.8.2 running FileMaker Pro 12v2 on an iMac. I created a new database file with a single field and a large value list of 15+ items, including "••••" in some of the values per your screenshot. I tested with the field both at the top and at the bottom of the layout with the zoom set to 150% and tested every avaliable monitor resolution on the iMac.
Are you able to make this happen in a new database file?
Can you provide more information about the hardware configuration?TSFalconFileMaker, Inc.