Similar to bug 1245815, textfield won't get repainted when both ends were
covered by other windows and then exposed. I have not verified with other
widgets, but I suspect other widgets, such as label, textArea, may have similar
problem as well.

The description field as copied from bug report 1245815 follows:

Given a scrollbar with both incremental arrows covered by other two
windows, when the scrollbar is exposed, the damaged area won't be
repaired.