If I set the Merge Strategy as "Fast-forward only" it should stop an out of date branch from being merged to master (Merge button should be disabled), however I am able to merge.
Steps to reproduce
* Clone two copies of a single repo and create two branches from master in each
* Set the merge strategy of the repo to "Fast-forward only" disabling all others
* Make independent changes on each repo and branch and push them to the remote
* Create two pull requests and merge one of them
* It should not be possible to merge the second pull request because the source branch is now behind master
I tried this against v5.12.2 (hosted BitBucket)
This used to work in older versions.