Description of problem:
Currently urwid has problems when a pile is nested into another one.
The problem was fixed for the ConfirmedEntry widget in 3.0 by handling that ui element, but this should be fixed generally so new widgets (which might be containers) can also be build by the urwid builder.
pushing to target release 3.5, assuming its not planned for 3.4 at this point...
Closing old bugs. If this issue is still relevant/important in current version, please re-open the bug.