“Update branch”/“Resolve conflicts” buttons should be present even if it is not possible to merge the pull request, for example, it’s a Draft pull request or the Reviewer selected “Request changes”.
First, it will show if the branch is out-of-date or there are any conflicts on the pull request.
Second, pressing this button will re-run automated checks (such as Travis)
I really love this new feature of draft pull requests (Thanks!!!) but it would be so much more useful if it allowed automatic branch updates!