[amjjbonvin]

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What does this PR do and why?

The changes job compares the whole PR with main. A commit that only resolves a CHANGELOG.md conflict after approval, or a merge of main into the branch, therefore reruns the full test suite (about 30 min, six jobs) although the code of the PR did not change.

• New step in changes: on a PR update (synchronize), compare the PR's own diff on the code paths (src, tests, integration_tests, end-to-end_tests, docs, pyproject.toml, setup.py, Dockerfile) at the previous and the new head, each against its merge base with main. git patch-id ignores line numbers, so merging main does not count as a change.
• Skip only when safe: the test and docker jobs are skipped only if the code is unchanged and all test-* check runs passed on the previous head, because GitHub treats skipped required checks as passing.
• Tests still run after a code change, when the previous run failed, was cancelled or did not run, when the previous head cannot be fetched (force-push), when the API call fails, for new PRs, and for scheduled and manual runs.
• .md files under the code paths (except docs/) no longer trigger the tests.

How was this tested?

The step's script, extracted from the workflow, was run locally with bash -eo pipefail on the commits of #1745:

• merge of main after a fully tested push: tests skipped;
• merge of main after a push on which no CI ran: tests run;
• code change (969093bfb): tests run.

The workflow itself can only be exercised after merging: this PR does not touch the code paths, so only the changes job runs here.

AI assistance

Claude Code wrote the workflow change based on my specifications and checked it as described above. I reviewed the changes.

Checklist

• Tests cover the new and/or changed code (CI only; tested locally as described)
• Documentation updated if needed (also in the haddock3 user-manual) (not needed)
• CHANGELOG.md updated for user-facing changes (not user-facing)

Related issues

Closes #1748

Notes for reviewers

• After merging main into a branch, the combination is no longer tested in the PR; it is tested on main by the scheduled run after the merge.
• After a skipped run, the next non-code push runs the tests again (the previous head has skipped, not passed, checks).

🤖 Generated with Claude Code