SRFI 159 says that under the description of "columnar" that "... and
all but the last column are right-padded."

The reference implementation, however, also pads the right column, at
least according to the following test:
https://github.com/ashinn/chibi-scheme/blob/master/lib/chibi/show-test.sld#L701

In fact, the sample implementation makes a difference between a
char-whitespace? pad-char and a non-char-whitespace? one:
https://github.com/ashinn/chibi-scheme/blob/master/lib/chibi/show/column.scm#L201

What was the original intention?

Marc