I tracked it down to the QuickAdd widget. I selected each item on the right column one by one moving them to the left column(uninstalling the item). When I got to QuickAdd it disappeared. Since I don't use that widget, it doesn't show the full dialog box. If I show that window, a box appears and right-clicking shows the menu cbprimula
showed in his post(mine is in in English).
I was beginning to think I had a dead pixel, but it was only dead on the desktop.
I now have only the five widgets I regularly use on the right column. Everything else has been moved to the left column. Problem solved. Maybe Rainy
can track down why it leaves an artifact pixel behind, even though the window is not actively displayed.
Hope that helps,