fix: compute replay mask rects in the same frame as the pixels

[posthog]

💡 Motivation and Context

• Who it hurts: every Flutter customer running maskAllTexts or maskAllImages can ship partially-unmasked replay frames whenever the UI moves during capture — the reporter confirmed masked health data was legible in paused recordings, which halted their production rollout. Affects 5.35.1 and 5.36.1, the default masking path, with no client-side workaround.
• Root cause (stale-mask race): captureScreenshot() snapshotted the mask rects synchronously up front, then the async body awaited a platform-channel round trip (isSessionReplayActive()) and endOfFrame before grabbing pixels with toImage(). The rects are absolute positions frozen at walk time, so frame N's black boxes get painted onto frame N+k's image. Sit still → looks fine. Scroll or navigate fast → content moved out from under the boxes and renders through.
• Fail-open second bug: a failed tree walk (walk returns null) silently shipped the raw, unmasked image.

💚 How did you test it?

• Not run manually. Flutter tooling is not available in this environment, so flutter analyze and the widget tests were not executed here — please run CI / a local reviewer pass before merge.
• Change is minimal and matches the pattern already used correctly by _collectPlatformViewRects() in the same file, which walks inside the async body.

📝 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

Changes

• Move both mask walks (getPostHogWidgetWrapperElements() and getCurrentWidgetsElements()) inside the async body, right after await endOfFrame with no await between them and toImage() — rects and pixels now come from the same frame.
• Fail closed: when maskAllTexts/maskAllImages is on and the walk returns null, drop the frame instead of shipping an unmasked screenshot.

🤖 Agent context

Autonomy: Fully autonomous

• Authored by Claude (Opus 4.8) via PostHog Desktop from an inbox report.
• Checked for competing work first: open PRs are deps and surveys (chore(deps): bump the codeql-action group across 1 directory with 2 updates #528, feat(surveys): add optional intro screen before the first question #515, fix(replay): keep session replay recording across a session change #510, feat(surveys): add displaySurvey to show a survey on demand #495); the one related issue (Session replay: apply remote (server-side) masking config #492, remote masking config) is unassigned and about a different feature. No overlap.
• Scoped the fix to the reported symptom only — the surrounding platform-view and dedup logic was left unchanged.

---

Created with PostHog Desktop from this inbox report.

[github-actions]

posthog-flutter Compliance Report

Date: 2026-08-18 14:19:00 UTC
Duration: 97290ms

✅ All Tests Passed!

45/45 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	159ms
Format Validation.Event Has Uuid	✅	135ms
Format Validation.Event Has Lib Properties	✅	130ms
Format Validation.Distinct Id Is String	✅	125ms
Format Validation.Token Is Present	✅	125ms
Format Validation.Custom Properties Preserved	✅	126ms
Format Validation.Event Has Timestamp	✅	126ms
Retry Behavior.Retries On 503	✅	5345ms
Retry Behavior.Does Not Retry On 400	✅	2126ms
Retry Behavior.Does Not Retry On 401	✅	2124ms
Retry Behavior.Respects Retry After Header	✅	8132ms
Retry Behavior.Implements Backoff	✅	15455ms
Retry Behavior.Retries On 500	✅	5234ms
Retry Behavior.Retries On 502	✅	5233ms
Retry Behavior.Retries On 504	✅	5234ms
Retry Behavior.Max Retries Respected	✅	15459ms
Deduplication.Generates Unique Uuids	✅	133ms
Deduplication.Preserves Uuid On Retry	✅	5233ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10343ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5242ms
Deduplication.No Duplicate Events In Batch	✅	135ms
Deduplication.Different Events Have Different Uuids	✅	125ms
Compression.Sends Gzip When Enabled	✅	124ms
Batch Format.Uses Proper Batch Structure	✅	121ms
Batch Format.Flush With No Events Sends Nothing	✅	113ms
Batch Format.Multiple Events Batched Together	✅	136ms
Error Handling.Does Not Retry On 403	✅	2122ms
Error Handling.Does Not Retry On 413	✅	2122ms
Error Handling.Retries On 408	✅	5235ms

Feature_Flags Tests

✅ 16/16 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	18ms
Request Payload.Flags Request Uses V2 Query Param	✅	16ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	16ms
Request Payload.Flags Request Omits Authorization Header	✅	16ms
Request Payload.Token In Flags Body Matches Init	✅	14ms
Request Payload.Groups Round Trip	✅	16ms
Request Payload.Groups Default To Empty Object	✅	15ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	16ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	15ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	14ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	14ms
Request Lifecycle.No Flags Request On Init Alone	✅	7ms
Request Lifecycle.No Flags Request On Normal Capture	✅	119ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	26ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	15ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	125ms