My vote here would be for PAN to simply remember what it was set for last.
If the subject column was very wide, keep it that way. If it was narrow,
keep it that way. Ideally, this info would be remembered per group, but
an overall global memory would be better than none, and better IMO than
dynamically resizing, as well.
While on the subject, it'd be nice to have old-PAN's ability to reorder
columns as well. Ideally, one could choose not to display certain columns
entirely. I don't use the score column that often, for instance, and
eliminating it would leave more room for the other columns. However,
allowing one to at least reorder seldom used columns to the far right,
then enlarge the subject and author columns so the unused ones are out of
the display range when horizontally scrolled left, works, and is the
solution I use on 0.14.x PAN. For it to work, however, (1) the columns
must be reorderable, and (2) PAN must remember the widths, so the stuff
scrolled off the right side of the display stays that way, and one doesn't
have to screw with resetting it each and every time.