Add selected line to relation of other selected line

[tordans]

Description

For tags, we have this nice feature that when I select multiple ways and edit the tags in the "Tags" section the change gets applied to both ways. I use this to sync tags between ways by selecting one segment that has the tags I need and one that does not. The tags for both are then visible as * (because they are not the same) but when I edit the value the UI shows what is present for the "source way" and applies it to the "target way".

I was trying to do the same for relations. But this part of the UI works differently:
When I select a way that has a relation and one that does not (yet)…

• … the dropdown does not show (only) the source-way relation(s) but all => it would be great if the drowdown now only showed what is there already; or at least put those that are present in one of the ways at the top or in a separate select group
• … then I re apply the relation, it gets added a second time to the source-way; which is now part of the relation twice (without warning)? => it would be great if in this case only the target-way would get the relation since the source way already has it

Example:

Screenshots

No response

[danieldegroot2]

This issue is a duplicate of (part of) #2152 .
I would suggest merging comments there.

[aditya1010-max]

I’ve opened a PR #11949 that addresses your first point about the relations dropdown.

Currently, the dropdown lists nearby relations, but it’s not clear which ones are already applied to the current selection. This PR shows that information in the UI.

Relations applied to all selected entities are marked as “Applied”
Relations applied to some selected entities are marked as “Partially Applied”

This does not prevent duplicate relation members from being created or change any relation logic. The goal is to make the existing behavior more visible rather than enforce new constraints.

More intrusive approaches (e.g. blocking duplicates or warning after selection) would likely require changes to action or membership logic and felt out of scope for me to take on in this first step.

Feedback welcome , especially on whether this direction makes sense.