ci: report frontend test coverage on pull requests

[mscolnick]

What & why

Frontend test coverage was never measured or reported anywhere. The backend already publishes coverage via Codecov (test_be.yaml); per maintainer decision the frontend gets PR comments only — no Codecov wiring for the frontend. This lands as part of the marimo-team engineering-excellence initiative to make code-health signals visible on every PR.

What was added

• frontend/vitest.config.ts — a coverage block (v8 provider, include: ["src/**"], reportOnFailure: true, reporters text, html, json-summary, json). It is opt-in: enabled is left unset so coverage only runs when --coverage is passed. This matches the pattern used across the org's codemirror repos.
• frontend/package.json — new test:coverage script (vitest run --coverage) and dev dep @vitest/coverage-v8 pinned to 3.2.4 to exactly match the locked vitest (avoids a peer-dependency mismatch). Lockfile updated (additive only).
• turbo.json — a test:coverage task mirroring test (dependsOn: ["build"], outputs: ["coverage/**"]).
• .github/workflows/test_fe.yaml — the test step now runs pnpm turbo test:coverage, and a SHA-pinned davelosert/vitest-coverage-report-action@…v2 step posts the coverage comment. The test_frontend job gains pull-requests: write.
• frontend/.gitignore — ignore coverage/.

Design choice: coverage in CI only

pnpm test / pnpm turbo test are unchanged — they still run plain vitest, so local runs and the default CI test path keep their current speed and behavior. Coverage instrumentation (which is not free on a repo this size) is gated behind the explicit --coverage flag via the separate test:coverage script, which CI invokes. reportOnFailure: true means the comment still appears when tests fail.

Fork-PR handling

marimo gets many PRs from forks, where GITHUB_TOKEN is downgraded to read-only and posting a comment 403s. The report step uses if: always() + continue-on-error: true so external contributors' checks never go red because of the comment (same pattern just adopted in marimo-lsp#634).

Local verification

• pnpm install — clean; only @vitest/coverage-v8 (+ its transitive deps) added to the lockfile; pnpm dedupe --check passes.
• Ran the CI coverage command in frontend/ on a subset (vitest run --coverage src/utils/__tests__/arrays.test.ts) — the full frontend suite is too slow to run to completion locally, so a subset was used to validate wiring and output path.
• Confirmed coverage/coverage-summary.json and coverage/coverage-final.json land in frontend/coverage/ — exactly where the workflow points (working-directory: ./frontend + coverage/coverage-summary.json, which the action resolves via path.resolve).
• oxlint and oxfmt clean on the changed config; git tree clean (coverage/ is now ignored).

Scope: only test_fe.yaml, the vitest/turbo config, package.json (+lockfile), and .gitignore. Backend workflows and Codecov config are untouched.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Jul 22, 2026 3:07pm

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[github-actions]

Coverage Report for ./frontend

Status	Category	Percentage	Covered / Total
🔵	Lines	76.82%	76787 / 99951
🔵	Statements	76.82%	76787 / 99951
🔵	Functions	71.42%	655 / 917
🔵	Branches	80.23%	4483 / 5587

File Coverage

No changed files found.

Generated in workflow #19787 for commit ef86898 by the Vitest Coverage Report Action

[cubic-dev-ai]

No issues found across 6 files

Architecture diagram

sequenceDiagram
    participant Dev as Developer (PR)
    participant GHA as GitHub Actions (test_fe.yaml)
    participant Turbo as Turbo (monorepo runner)
    participant Vitest as Vitest w/ Coverage
    participant FS as Coverage Files (coverage/*.json)
    participant ReportAction as vitest-coverage-report-action
    participant GH_API as GitHub API (PR Comment)

    Note over Dev,GH_API: PR Coverage Report Flow

    Dev->>GHA: Push PR / update branch
    GHA->>GHA: Trigger test_frontend job
    Note over GHA: permissions: pull-requests: write

    GHA->>Turbo: NEW: pnpm turbo test:coverage
    Turbo->>Turbo: dependsOn: ["build"] (cached or fresh)
    Turbo->>Vitest: vitest run --coverage
    Note over Vitest: Coverage only runs with --coverage flag (opt-in)
    Vitest->>FS: Write coverage/coverage-summary.json, coverage-final.json, etc.
    Note over Vitest: reportOnFailure: true → written even on test failure

    alt Tests pass or fail (always())
        GHA->>ReportAction: NEW: Run davelosert/vitest-coverage-report-action
        Note over ReportAction: continue-on-error: true
        ReportAction->>FS: Read coverage JSON files
        FS-->>ReportAction: summary & final coverage data

        alt Fork PR (GITHUB_TOKEN read-only)
            ReportAction->>GH_API: POST PR comment
            GH_API-->>ReportAction: 403 Forbidden
            Note over ReportAction: continue-on-error → check not marked red
        else Non-fork PR (GITHUB_TOKEN write)
            ReportAction->>GH_API: POST PR comment
            GH_API-->>ReportAction: 201 Created
            Note over GH_API: Coverage report appears on PR
        end
    end

    Note over GHA: Optional: Summarize typecheck failure (unchanged)

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[github-actions]

🚀 Development release published. You may be able to view the changes at https://marimo.app?v=0.23.15-dev55