To reproduce (tested on Mac OS X 10.6, Cocoa)

1. Create a new widget in Designer
2. Add a list widget
3. Add an item to the list widget which is wider than the viewport, so that the list widget gets a horizontal scrollbar
4. Use the scrollbar to scroll back and forth while in edit mode

You should see that little pieces of the blue size grips are not erased, blue artifacts all over the list view.