move single renamed EXDEVed files

If a rename will result in a EXDEV with a path preserving policy: attempt to replicate what mv would do but only for single files. Should cover a decent percentage of situations. If not perhaps look into full mv behavior replication.

I was going to request for mv to actually use the same device whenever possible. Is this somehow related?

No. This is to completely hide the underlying fact that a rename across drives can result in EXDEV when using a path preserving policy. IE. It'd do what mv is doing when the situation arises so that apps that aren't written well (many of them) hopefully won't freak out as they may now when they receive an EXDEV error.

Define 'whenever possible.' mergerfs already tries really hard to rename/link on the same device given the policy restrictions. It's actually the most complicated code within mergerfs. If a non-path preserving policy is used then it should never not succeed. And with path preserving you can use ignorepponrename to ignore the path preserving only on rename and link.