fix: guard Output page against stale async chart renders (#101)

[Sanjay Singh (san360)]

Summary

Fixes #101 — the Output page's intermittent ⚠️ Failed to render Output — Cannot read properties of undefined/null (reading …) error boundary crash.

Root cause

createChart() in src/webview/shared.ts passed document.getElementById(canvasId) straight into new Chart(...) with no null check. renderOutput/renderProductionTab/renderTokenUsageTab in src/webview/page-output.ts render a spinner, await rpc(...), then re-render tab markup and call createChart(...) — with no cancellation of stale in-flight renders across that async gap.

If the user switched nav pages, toggled the Code Output ↔ Token Usage tab, or clicked date-range buttons rapidly while a request was still in flight, the stale promise would eventually resolve and either:

• find its expected canvas missing from the live DOM (Chart.js dereferences the null element and throws), or
• find the canvas still present but belonging to a different tab that has since been rendered, silently clobbering it with stale data.

Fix (two layers, per the repro/fix notes in #101)

1. createChart (src/webview/shared.ts) — now checks document.getElementById(canvasId) for null and returns null with a console.warn instead of handing a null element to new Chart(...). Return type updated to Chart | null; no call site used the return value, so this is a safe, non-breaking change.
2. page-output.ts — added a module-level renderGeneration counter, bumped once per render attempt (initial mount, tab switch, or range change) in renderActiveTab(). renderProductionTab/renderTokenUsageTab capture the generation at entry and bail out immediately after their await rpc(...) if a newer render has since started, so a slow/superseded response never touches the DOM again.

Testing

Ran the full repo check suite from the worktree:

• npm run typecheck ✅
• npm run lint ✅ (0 errors; pre-existing warnings only, none introduced)
• npm run spellcheck ✅
• npm run knip ✅
• npm test (vitest) — 1341 passed; 7 failures in src/core/github-app-analytics.test.ts are pre-existing and unrelated (verified by stashing this change and re-running the same file — identical failures on main).
• npm run build + npx playwright test (full e2e suite) — 37 passed, 7 skipped (pre-existing feature-flag gating), 0 failures, including all applicable tests/e2e/output.spec.ts cases.

Notes

• No dependency, config, or public API changes.
• createChart's new Chart | null return type doesn't affect any existing caller (none of the ~30 call sites across the webview capture the return value).

[github-actions]

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Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None