Upon examination, the line is indeed created by the toolbar. Furthermore, the toolbar is always placed at 0,0 regardless of the coords given when it's created. This means that multiple toolbar gadgets "fight" each other, since they can't be placed anywhere but with the same origin:
The above image shows what happens when two toolbars are created, one at 0,0 and the second at 500, 100. The coordinates are ignored and the second toolbar's left/right arrows appear superimposed on the first toolbar's New/Open icons (both toolbars are PNGs with transparency)
The above image shows what happens when two toolbars are created, one at 0,0 and the second at 500, 100. The coordinates are ignored and the second toolbar's left/right arrows appear superimposed on the first toolbar's New/Open icons (both toolbars are PNGs with transparency)