Branch name of workflows does not accurately display the branch name of the workflow file executed

[ChipShaddAtCube]

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Bug

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I noticed some weird behavior while troubleshooting something else. I will include what I was troubleshooting in addition to what I found.

My use case:

• Run an action after a PR has been merged

Core Workflow file

on:
  pull_request:
    types:
      - closed
    branches:
      - main

jobs:
  run-action:
    if: github.event.pull_request.merged == true
    runs-on: ubuntu-latest
    steps:
      - name: Checkout repository
        uses: actions/checkout@v2

Expected behaviour

• GH Actions to pull down newly merged code in main
• GH Actions to use the workflow file in main
• GH to show the main branch in the list of previous workflow runs

Actual behavior

• GH Actions DOES pull down the newly merged code in main
• GH Actions uses the workflow file from the source branch
• GH Actions shows the source branch in the list of workflow runs
  • Although it should not be an issue, it is confusing as it looks like the action was run on code from the source branch and not the main branch
  • I have other hooks that run on push to main so in the list of workflow runs, users expect to see that all actions were run on main, but instead, they see that hooks were run on main, and code was seemingly deployed from the feature branch.

The weird behavior

This got me thinking...what if the workflow in main was updated, and a feature branch was merged in with an outdated workflow file (not best practice, I know).

In my main branch, I cut a feature branch, and then I changed the trigger to the following, directly in the main branch:

on:
  push:
    branches:
      - main

jobs:
  run-action:
    if: github.event.pull_request.merged == true
    runs-on: ubuntu-latest
    steps:
      - name: Checkout repository
        uses: actions/checkout@v2

I then merged in the feature branch, without rebasing, into main. (feature branch's workflow file still set to trigger on PR closure).

Expected behavior

• GH Action will use the outdated workflow file from the incoming feature branch (as established above)

Actual behavior

• GH Action used the workflow file in main
• GH Action was skipped due to the failed if: github.event.pull_request.merged == true conditional in main
• On the skipped run, when examining the workflow file used, GH showed the workflow file of source feature branch as well as the commit hash which showed the workflow trigger was still set to run on PR closure.

It seems like GH uses the most recent version of the workflow file between the branches being merged? Although the UI (in multiple places) indicate otherwise.