After scrolling in either data view or layout view, some parts of the ArcMap display do not draw or refresh completely.
There are many reasons why the display may not refresh completely. This is usually caused by interrupting the redraw while scrolling. If you click to scroll, and then click to scroll again while the display is still redrawing it may not refresh completely. Other causes may include network latency, low memory, conflicts with third-party tools, etc.
Manually refresh the display by clicking on the refresh button or by pressing the F5 key. If this does not help, see the web help topic, Tips on improving map display performance. See also the GeoNet blog: Troubleshooting Slow Performance in ArcGIS for Desktop