@dschinkel ,

Once a PR is merged, it will be closed automatically. The activity type ‘closed’ of the pull_request event should work.

In your case, you can reference to the example workflow below to configure yours:

on:
  pull_request:
    types: [opened, synchronize, closed]
    branches:
      - master
jobs:

build:
Some jobs do not have any if statement.
They will always be executed in the workflow. For example, 'build' job.
test:

if: ${{github.event_name == 'pull_request' && github.event.action != 'closed'}}
With this if statement, the 'test' job will only run
when the activity type of the PR is not closed.

Of course, you also can remove ‘closed’ from the list of the activity types, so that the workflow will not be triggered every time the PR is closed (manual or merged).

on:
  pull_request:
    types: [opened, synchronize]
    branches:
      - master