ci(positron): run Positron API tests on ubuntu-latest

[jonvanausdeln]

Moves the Positron-API-Tests workflow from macos-latest to ubuntu-latest. Linux Actions minutes bill at roughly 1/10 the macOS rate.

Part of posit-dev/positron#15281. The suite itself landed in #4298.

Changes

Beyond the runner line, four things were required:

1. xvfb-run -a wrap. Positron is an Electron app and needs a display server on Linux. posit-dev/setup-positron is a thin composite step that only runs the command it is handed, so it provides none. Without this the run fails with Missing X server or $DISPLAY and then dies with SIGSEGV.
2. @posit-dev/positron-test-electron → ^0.0.3. 0.0.2's platform table is darwin-arm64/darwin-x64 only; linux-x64 and win32-x64 descriptors land in 0.0.3 (the latest published version). The bump is in extensions/vscode/package.json with the root workspace package-lock.json updated to match.
3. posit-dev/setup-positron@v1 instead of @main, so upstream changes to the action can't break CI here.
4. ${{ runner.os }} in the Positron download cache key, on both the restore and the save step. The cache layout is .positron-test/<channel>/<version>/<platform>, so under an OS-less key a stale same-day macOS entry would restore, set cache-hit=true, suppress the save step, and the Linux build would never get cached that day.

Also:

• Added workflow_dispatch alongside workflow_call so the suite can be triggered manually (e2e.yaml already does both). This is what made the pre-merge verification below possible.
• Replaced the "macOS only" notes in scripts/run-positron-tests.mjs, src/test/positron/README.md, and extensions/vscode/CLAUDE.md with the xvfb-run recipe for headless Linux runs. Unrelated darwin platform switches in extension source are untouched.

Verification

Dispatched on a fork before opening this PR (run, commit 4b9ac8d8). A green job alone doesn't prove the tests ran, so from the log:

Run xvfb-run -a npm run test-positron
  ✔ Positron injects the acquirePositronApi global
  ✔ Publisher activates in Positron
  ✔ resolves the Python interpreter from Positron's preferred runtime (2051ms)
  ✔ resolves the R interpreter from Positron's preferred runtime
  4 passing (3s)
Extension host test runner exit code: 0
Exit code:   0

Both interpreter-discovery tests pass, so actions/setup-python@v7 and r-lib/actions/setup-r@v2 resolve on ubuntu and Positron's runtime discovery finds both. positron-channel: stable works on Linux — the 0.0.3 README's "stable not available on Linux/Windows yet" caveat is stale. Wall clock was ~5 minutes.

No CHANGELOG entry: CI-only, no user-facing behavior change.

[posit-connect-projects]

Claude finished @jonvanausdeln's task in 58s —— View job

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Code Review Complete

• Read changed workflow file
• Read documentation and script changes
• Review package.json and lockfile changes
• Check git diff for complete changes
• Provide code review feedback

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Review Summary

This is a well-executed PR that moves the Positron API tests from macOS to Ubuntu Linux for significant cost savings. The changes are thorough and address all the platform differences correctly.

What looks good

1. Cache key fix (.github/workflows/positron-api-tests.yaml:34,47): Adding ${{ runner.os }} to both the restore and save cache keys is essential. Without this, a macOS cache entry could be restored on Linux, causing the save step to be skipped (cache-hit=true), leaving Linux builds uncached.

2. xvfb-run wrapper (line 42): Using xvfb-run -a is the correct approach for headless Electron apps on Linux. The -a flag automatically finds a free display number, avoiding conflicts in concurrent runs.

3. Action version pinning (line 37): Switching from @main to @v1 is a good stability improvement — protects CI from unintentional breaking changes upstream.

4. Package bump (extensions/vscode/package.json:955): The @posit-dev/positron-test-electron bump to ^0.0.3 is required since 0.0.2 only had darwin-arm64/darwin-x64 platform descriptors.

5. workflow_dispatch addition (lines 2-4): Enabling manual dispatch is useful for debugging and pre-merge verification, matching the pattern in e2e.yaml.

6. Documentation updates: All three docs (CLAUDE.md, README.md, run-positron-tests.mjs) are updated consistently to replace the "macOS only" notes with the xvfb-run recipe.

No issues found

The implementation is clean and complete. The verification run on a fork with visible test output in the logs confirms the tests actually executed. No CHANGELOG entry is correct per the repo guidelines (CI-only, no user-facing change).

Verdict: Approve — Ready to merge.

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