Fix another glitch with on-screen indentation.

Fix another glitch with on-screen indentation.

If a new paragraph is created just before a nested environment,
the indentation of the nested environment is not computed
correctly because the parindent of the previous layout would
also be erroneously taken into account. This would cause the
nested environment to move back and forth when something is
added to the new paragraph.

A proper status line covering this change is already present.