The arrow keys and space in develop called `next-image` and
`prev-image`, which walk the files given on the command line. A
photograph opened from a library leaves that list empty, so the keys did
nothing and only a click on the photo roll moved on.
With a library open the step now goes through the roll: it opens the
neighbouring frame exactly as clicking it would, and saves the outgoing
edit the same way.
dtourolle
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