GitLab has useful feature - it displays the following message:
The source branch is <n> commits behind the target branch
in case when new commits were pushed to target branch.
It would be great if GitHub provides some analogue.
Just sometimes new commits in target branch might create conflicts with PR, not only in code itself (so git merge executes successfully), but, for example in API (new commits contains refactoring, while PR uses old API in new code).
Such message at least can be used as a warning for reviewer and submitter.