Changing the branch of topic changesets leaves MR stranded
If one makes a MR from a topic (), and afterwards one changes the named branch of that topic (), from the MR point of view, the source branch has disappeared, and there does not seem to be a way from the UI to change it.
Of course, the topic can still be published (in the new named branch, then), but the only thing users can do at this point is to close the MR, as if it's been rejected.
Example in Heptapod case: !32 (closed)
I'm not sure what would be a good solution, functionally speaking, and it won't probably be worth the effort to fix that for a long time.
(*) this is a simplification, what appears as GitLab branch is a topic/named branch combination