fix(frontend): prevent replay capture from freezing maps

[JesJehle]

Briefing

PostHog replay canvas capture was synchronously reading every OpenLayers canvas several times per second, which caused repeated main-thread stalls while users panned or zoomed maps. This change keeps useful map imagery in session replays while replacing continuous capture with bounded settled and periodic frames across the normal DeadTrees and PRIWA maps. Non-map canvases are excluded so camera-backed and processing canvases cannot enter replay.

What changed

• Limit eligible map frames to 1 FPS, 50% linear resolution, and 0.3 encoding quality.
• Suppress map canvas reads during pointer and wheel interaction, then capture one settled frame after the interaction burst.
• Refresh a newly loaded map after 5 seconds and then every 30 seconds so asynchronous tile, layer, and programmatic changes do not leave replay stale.
• Fail closed for every canvas outside an OpenLayers viewport, including the hidden QR scanner canvas.

Validation

• npm --prefix frontend test: 66 files, 329 tests passed.
• npm --prefix frontend run lint: passed.
• npm --prefix frontend run build: passed.
• scripts/lint-ast-grep.sh: passed.
• Prod-connected local normal map baseline, 60 sustained gestures: 549 canvas reads, 3 long tasks, 158 ms maximum frame gap.
• Same normal map with canvas recording disabled: 0 canvas reads, 0 long tasks, 48 ms maximum frame gap.
• Final PRIWA mobile-sized run, 10 sustained gestures: 0 canvas reads during interaction, 1 settled 197x426 frame afterward, 0 later idle reads in the observation window, 0 long tasks, and 0 frame gaps above 20 ms.
• Production PostHog diagnostic, last 14 days: 57 map sessions had replay snapshot stalls of at least 500 ms across mobile and desktop traffic.
• Thermonuclear review completed with no remaining P1; Codex autoreview ran twice and both accepted P1 findings were fixed and locally verified.

Risk and limitations

Map replay imagery is intentionally lower fidelity and temporally sparse. Non-map canvases are no longer recorded; ordinary DOM replay, pointer activity, and analytics events remain available. A bounded map refresh can still do one canvas read after interaction settles, but continuous capture during active panning is removed.

Reviewer guide

Please focus on the maskRegionsFn contract: null skips a frame, while [] permits only an explicitly selected OpenLayers map frame.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-02T08:29:35.108359Z	4d27aea	PR opened

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Channel: pr572-fix-posthog-map-canvas-r
Commit: 4d27aea1aa18a376ed70fb7f01b3d83a42ffe06f