JOSM Conflict window.

I'm still a bit confused.

There's 2 states, the latest server version (their)

OK

and the local version (my).

The state of the database after the changeset edit, yes?

The version you downloaded is also on the server, as a previous version (but it's not tracked for the conflict).

The state of the database before the changeset edit, yes?

The pale items are present in both versions but not in the same position.

OK

There's some help on the JOSM wiki

Which, after reading, I needed to post this question.

There's 2 states that are relevant to the conflict. "their" and "my" have both replaced it…it's just that one of them has done it on the server and one hasn't.

@DaveF the dialogue was not created with reversion in mind. I think the dialogue assumes that what happened was this:

1. You downloaded some data and started a long edit session
2. Someone else submits a changeset involving data you've also edited.
3. You attempt to upload your changes but some of the stuff you changed has already been changed on the server. The server rejects the changes because you are trying to make a change based on an out of date version of the object.
4. JOSM tries to figure out if both sets of changes can co-exist, fails to do this as they are contradictory and asks you which bits of your update (my) or the other mappers update (their) should be uploaded.

In some cases you may both have been refining distant areas of the same coastline so it makes sense to keep elements of both lists of nodes. In other cases you may have both tidied the same building to slightly different imagery in which case a merge would only re-introduce distortion. In the case of a reversion you probably need to determine whether the other mappers edits are an improvement on what was there before the changeset you're trying to undo.

You could use achavi to assist in seeing the other persons changes since you started editing. Zoom to the area, select bbox and draw a rectangle around the area, adjust the 'start date and time' entry box if necessary and select load. This will give you a clearer understanding of what 'their' edits were.