fix(router): stop reporting browser network failures from route loaders

[posthog]

Problem

• Error tracking gets new noise issues when a user leaves a page while a route loader still fetches. The user sees nothing wrong.
• The workspace loader calls a server function through ensureQueryData. When the browser cancels that request (for example on a full-page navigation), fetch rejects with a bare TypeError: Failed to fetch.
• The router defaultOnCatch sends every loader error to PostHog, so this network TypeError becomes an error-tracking issue.

Origin

• Error tracking: issue
• First signal: 2026-09-28
• Inbox report: open
• Likely cause: 6b4e1da
• Task started by: auto-start, after the report was rated P4 and ready to fix

Changes

• defaultOnCatch now skips capture for TypeErrors that have a known browser network-failure message.
• New helper isNetworkError in src/lib/network-error.ts holds the message list:

Engine	Message
Chromium	Failed to fetch
Firefox	NetworkError when attempting to fetch resource.
Safari	Load failed

• The match is exact and needs a TypeError. Other TypeErrors (for example Cannot read properties of undefined) still go to error tracking.

Note

The error screen behavior does not change. A real network drop still shows AppErrorScreen. Only the report to error tracking stops.

Testing

• vitest --project=node src/lib/network-error.test.ts: 2 tests pass.
• vp check on the changed files: no format, lint, or type errors.

Agent context

• I did not add a loader retry. fetchQuery does not retry by default. A retry does not help a request that the browser cancels during unload, and it makes a real outage show the error screen later. We can add it as a separate change if real network drops cause user complaints.
• I put the filter in the router only, not in capturePostHogClientException, so the other capture paths keep their current behavior.

Closes #845

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Created with PostHog Desktop from this inbox report.

🤖 Generated with Claude Code

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[github-actions]

React Doctor found no new issues. 🎉

_{Reviewed by React Doctor for commit fe49390.}