The screen does not redraw correctly after scrolling. Most noticeable in form view. Things will be misaligned and overlapping on top of each other. This issue was not present in Filemaker Pro 11 that I upgraded from.
When you open the Windows Control Panel for your display settings, is 100% selected? If not, please select 100% and see if that corrects this issue. There are "known issues" caused when using FMP 13 on a windows 7 or newer system with a display setting other than 100%.