fix(replay): web canvas masking keeps an app-provided maskRegionsFn for non-Flutter canvases

[turnipdabeets]

💡 Motivation and Context

Follow-up to greptile's security note on #507 about code merged in #499: opting into web canvas masking unconditionally replaces session_recording.canvasCapture.maskRegionsFn with the provider's own callback, and the provider answers [] for any canvas outside a flutter-view. On a hybrid page whose original maskRegionsFn masked its own (non-Flutter) canvases, those canvases were recorded unmasked — the app's masking was silently dropped.

Now the provider captures the app's callback once, before the first set_config replaces it, and delegates canvases outside every flutter-view back to it. Its own flutter-view canvas keeps the widget-tree answer, a foreign flutter-view canvas still fails closed, and a throwing app callback fails closed too. One documented caveat: js_interop conflates undefined and null on the way back through Dart, so an app answer of undefined (record unmasked) is coerced to the fail-closed null — over-masking, never a leak.

💚 How did you test it?

• Four new browser tests: app callback's array answer passed through for a no-host canvas, its null kept, a throwing callback fails closed, and a second setup() does not re-capture the provider's own installed callback (which would recurse).
• Full battery: 247 VM tests + 62 browser tests pass, flutter analyze and dart format clean.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

• Claude Code session; scope came from greptile's finding on fix(replay): honor changed masking flags on iOS/Android across setup() calls #507 (which had reviewed a stale pre-rebase head — the flagged code is feat(replay): mask canvas session replay recordings on Flutter web #499's, on main). Verified the leak path before fixing: _unsafeComputeMaskRegions returned [] for host-less canvases, which posthog-js treats as "answered, zero regions".
• Capture-once static (rather than function-identity comparison) prevents a second setup() from capturing the provider's own installed callback — toJS wrapper identity is not reliable across compilers.
• Foreign flutter-view canvases deliberately keep failing closed rather than delegating: the app's callback was not written for another engine's canvas.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(replay): keep the app's maskRegionsF..." | Re-trigger Greptile}

[github-actions]

posthog-flutter Compliance Report

Date: 2026-07-30 16:13:39 UTC
Duration: 96795ms

✅ All Tests Passed!

45/45 tests passed

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Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	139ms
Format Validation.Event Has Uuid	✅	117ms
Format Validation.Event Has Lib Properties	✅	116ms
Format Validation.Distinct Id Is String	✅	115ms
Format Validation.Token Is Present	✅	114ms
Format Validation.Custom Properties Preserved	✅	116ms
Format Validation.Event Has Timestamp	✅	117ms
Retry Behavior.Retries On 503	✅	5331ms
Retry Behavior.Does Not Retry On 400	✅	2118ms
Retry Behavior.Does Not Retry On 401	✅	2118ms
Retry Behavior.Respects Retry After Header	✅	8121ms
Retry Behavior.Implements Backoff	✅	15433ms
Retry Behavior.Retries On 500	✅	5225ms
Retry Behavior.Retries On 502	✅	5227ms
Retry Behavior.Retries On 504	✅	5224ms
Retry Behavior.Max Retries Respected	✅	15436ms
Deduplication.Generates Unique Uuids	✅	125ms
Deduplication.Preserves Uuid On Retry	✅	5224ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10335ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5226ms
Deduplication.No Duplicate Events In Batch	✅	125ms
Deduplication.Different Events Have Different Uuids	✅	115ms
Compression.Sends Gzip When Enabled	✅	115ms
Batch Format.Uses Proper Batch Structure	✅	113ms
Batch Format.Flush With No Events Sends Nothing	✅	108ms
Batch Format.Multiple Events Batched Together	✅	122ms
Error Handling.Does Not Retry On 403	✅	2115ms
Error Handling.Does Not Retry On 413	✅	2118ms
Error Handling.Retries On 408	✅	5226ms

Feature_Flags Tests

✅ 16/16 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	12ms
Request Payload.Flags Request Uses V2 Query Param	✅	10ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	9ms
Request Payload.Flags Request Omits Authorization Header	✅	9ms
Request Payload.Token In Flags Body Matches Init	✅	9ms
Request Payload.Groups Round Trip	✅	10ms
Request Payload.Groups Default To Empty Object	✅	9ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	10ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	9ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	10ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	9ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	113ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	16ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	9ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	115ms

[posthog]

🦔 ReviewHog reviewed this pull request

Nothing worth raising this time, so here's a calming picture instead: