Merge layer: specific order of layers needed to find conflicts

[skyper]

I found this in #8948, see my comment 3.

What steps will reproduce the problem?

1. open attached file​ (Layer A)
2. select all objects
3. revert selection from changeset 16833418.
4. duplicate layer A (-> Layer B)
5. update data of layer A
   • get 28 conflicts (13 ways, 15 nodes)
6. download the area​ to new layer (Layer C)
7. merge layer B on layer C

What is the expected result?

Conflicts after 3. or at least after 7..

What happens instead?

No conflicts and no object is modified after 7.. Changes of layer B are silently dropped.

Please provide any additional information below. Attach a screenshot if possible.

I only get conflicts with "update data" but there should be conflicts or is this a (new) feature. At least no changes and no conflicts after 7. seems strange.
Merging Layer C on Layer B creates the expected conflicts.

Reverter plugin used to create conflicts but I think it is a new feature if properly documented if it does not.

In my eyes, merging layers needs to create conflicts dependent-less on the order as unlike conflicts with server data both layers can have contradicting changes and both are "my" versions.

I remember to have had similar issues when working offline with small overlapping extracts and then merging several modified extracts before upload.

Relative:URL: ^/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2020-10-29 00:51:28 +0100 (Thu, 29 Oct 2020)
Revision:17279
Build-Date:2020-10-29 02:30:54
URL:https://josm.openstreetmap.de/svn/trunk

Plugins:
+ reverter (35579)

[GerdP]

When merging layers I always miss the ability to say something like "if in doubt, prefer this layer" or maybe "considers this layer as mine and the other as theirs". Maybe that's because I don't know how it works in other programs. For me, multiple layers edit layers just mean double trouble, so I try to avoid them. But my case is very special because I often use JOSM to analyse special cases where I have a layer with a huge amount of objects and try to extract the data that is needed to reproduce a problem in mkgmap or JOSM.
Edit: Just understood it. The dialog asks for a "target layer". The source in DatasetMerger uses "target" and "source" and when you download data from server it will be the source and the existing (possibly empty) layer will be the target.