Found with 1.9.5 and still there in r14032:
When I reconcile and click on items so they get their check mark,
sometimes the whole dialog window is rebuilt. It looks like gnucash
decides that a column width is too small and makes it bigger (only
a few pixels I think), and that affects all columns to the right
of it.

Usually, it ends up looking nearly the same as before, but there's
a second of unrest while the columns are rebuilt.

Looking more closely, it looks like the values in the Amount column
jump about two letters wide to the right and then immediately back
to the left again, ending up a bit more to the right than before.
Then the column headers are moved to the right. Effectively, it
looks like the Description column gets a few pixels wider (less
than ten, perhaps three or four?). This is when I confirm an
entry in the "Funds In" part.

If I confirm an entry in the "Funds out" part, still the "Funds
In" Description gets wider, but less than when I confirm a "Funds
In" entry.

This is with gtk2+-2.8.17, pango-1.12.2, atk-1.11.4, glib2-2.10.2,
in case it matters.

David Hampton reported on gnucash-devel that he also sees this problem.