Prevent merging pull requests to master from outdated branches

[bielertc]

Scenario: Someone creates a pull request from branch foo to master. The automatic tests on this pull request pass. Before the pull request from foo is reviewed and merged, the master is updated independently. The changes to master don't create merge conflicts, so the Merge button remains available. However, when performing the automatic merge, this leads to a broken application. Now the automatic tests fail on the master branch.

Is there a way to disable the merge button (or show a warning) from any PR to master, if the branch that is being merged is not up to date with master?

[borN_free]

If anyone found a plugin, please share. Thank you!