To reproduce the problem, run the attached example using -style cleanlooks.

Two dock widgets, containing QTextEdits appear. When one is aligned with the default alignment and the other uses horizontal alignment, the rightmost dock widget will have a shadow border with the same effect as the cleanlooks splitter - i.e. fine lines.

The problem can be worked around by ensuring that the last call to addDockWidget does not contain the Qt::Horizontal flag, hence adding a final widget and then deleting it will make the problem disappear.