fix(client): filter Fathom removeChild-on-null Sentry noise (KODY-CLOUDFLARE-3Q)

[kentcdodds]

Summary

Filters browser Sentry noise from KODY-CLOUDFLARE-3Q (7653117289).

Fathom Analytics (cdn.usefathom.com/script.js) inserts a temporary <img> beacon and removes it in load/error handlers via img.parentNode.removeChild(img). After SPA navigation the beacon can already be detached, so parentNode is null and Chromium throws:

TypeError: Cannot read properties of null (reading 'removeChild')

Sentry's browserapierrors.addEventListener wrapper captures that third-party throw. Not a Kody product bug and not reasonably fixable in app code (external script).

Change

• Add filterFathomRemoveChildNullSentryEvent in sentry-browser-filters.ts
• Match requires both the Chromium removeChild-on-null TypeError text and a stack frame whose hostname is cdn.usefathom.com (so real app removeChild bugs still report)
• Wire into filterBrowserSentryEvent; unit tests cover drop / keep / hostname lookalike cases

Outcome

filtered — ship this filter, then ignore the Sentry issue so it stops paging.

No sibling unresolved issues share this Fathom / removeChild signature.

System recap — composes existing primitives (low risk)

Mode: recap · Base: main @ bd33593e · Head: 2888b905

Classification: composes — narrow browser beforeSend filter on the existing client Sentry path; no new primitives or contracts.

Primitives touched

Primitive	Group	Impact
app-ui	surfaces	composes — extends client Sentry noise filter list only

System map

Browser errors still flow through the existing client Sentry init; this PR only drops one third-party Fathom signature in beforeSend.

Legend: green = composes (wiring only) · amber = extended by this PR · red = new primitive · gray = context (unchanged, included only when an edge crosses it).

flowchart LR
	appUi["app-ui<br/>Browser app (Remix 3)"]:::touched
	sentry["Sentry browser SDK<br/>beforeSend"]:::untouched
	appUi -->|"filterFathomRemoveChildNullSentryEvent"| sentry
	classDef touched fill:#1a7f37,color:#fff
	classDef extended fill:#9a6700,color:#fff
	classDef added fill:#cf222e,color:#fff
	classDef untouched fill:#57606a,color:#fff

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Summary by CodeRabbit

• Bug Fixes
  • Reduced browser error noise by filtering a specific Fathom Analytics removeChild error.
  • Continued reporting similar application errors and unrelated analytics errors normally.
  • Improved browser error filtering for events with nested exception stack frames.
  • Added coverage to help ensure valid browser errors remain visible in monitoring.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 80e9ed93-91c4-436c-9eda-2e2946470244

📥 Commits

Reviewing files that changed from the base of the PR and between 733a3f4 and 2888b90.

📒 Files selected for processing (3)

• packages/worker/client/sentry-browser-filters.node.test.ts
• packages/worker/client/sentry-browser-filters.ts
• packages/worker/client/sentry-init.ts

🚧 Files skipped from review as they are similar to previous changes (3)

• packages/worker/client/sentry-browser-filters.node.test.ts
• packages/worker/client/sentry-init.ts
• packages/worker/client/sentry-browser-filters.ts

---

📝 Walkthrough

Walkthrough

Changes

The browser Sentry filters now inspect nested exception stack frames for URLs. They discard the exact null-removeChild error only when a Fathom CDN frame is present. Tests cover matching and non-matching events, and the Sentry comment documents the case.

Fathom Sentry filtering

Layer / File(s)	Summary
Stack-frame URL detection packages/worker/client/sentry-browser-filters.ts	Sentry event types and exception traversal now expose stack-frame URLs from nested exceptions.
Fathom filter integration packages/worker/client/sentry-browser-filters.ts, packages/worker/client/sentry-init.ts	The filter matches the exact null-removeChild error with a Fathom CDN frame and applies it through the browser event filter. The Sentry filter comment records the ignored error.
Fathom filter validation packages/worker/client/sentry-browser-filters.node.test.ts	Tests verify that matching Fathom events are removed and similar or unrelated events remain.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• kentcdodds/kody#1006: Modifies the same Sentry browser filter module for a different error type.
• kentcdodds/kody#1164: Adds a distinct browser-noise filter through the same combined filter path.
• kentcdodds/kody#1166: Adds and integrates another targeted browser Sentry-noise filter.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: filtering Fathom removeChild-on-null Sentry noise.
Description check	✅ Passed	The description explains the intent, changes, outcome, and testing coverage, with system impact documented in the recap.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch cursor/sentry-triage-kody-cloudflare-kody-cloudflare-3q-a817

---

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[github-actions]

🔎 Preview deployed: https://kody-pr-1232.kody-a99.workers.dev

Worker: kody-pr-1232
D1: kody-pr-1232-db
KV: kody-pr-1232-oauth-kv

Mocks:

• cloudflare: https://kody-pr-1232-mock-cloudflare.kody-a99.workers.dev/__mocks (kody-pr-1232-mock-cloudflare)

[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/worker/client/sentry-browser-filters.ts`:
- Around line 221-229: Update isFathomAnalyticsStackFrameUrl to parse the input
URL and return true only when its hostname equals cdn.usefathom.com, rather than
matching a URL substring; preserve safe handling for invalid URLs. Add a
regression case covering a non-Fathom hostname using /cdn.usefathom.com/ in its
path.

In `@packages/worker/client/sentry-init.ts`:
- Around line 32-36: Update the issue identifier in the Sentry filter comment
near filterBrowserSentryEvent from KODY-CLOUDFLARE-23 to KODY-CLOUDFLARE-3Q,
leaving the surrounding references unchanged.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: c5c5ec58-91b5-4f22-bba0-94e71622ffb8

📥 Commits

Reviewing files that changed from the base of the PR and between bd33593 and 733a3f4.

📒 Files selected for processing (3)

• packages/worker/client/sentry-browser-filters.node.test.ts
• packages/worker/client/sentry-browser-filters.ts
• packages/worker/client/sentry-init.ts