This will cause layers above `FoldMap` to grab a fresh snapshot of the `FoldMap`
and, as a result, of the underlying `MultiBufferSnapshot`. It is a necessary change
because, even though the coordinate space is not affected, a buffer edit taking place
*before* an excerpt range could cause the excerpt buffer rows to change, e.g. if
lines were added or removed. This manifested itself in a randomized test.
Antonio Scandurra
c0bf4a5bfd