feat(surveys): add optional intro screen before the first question

[jakesciotto]

💡 Motivation and Context

Surveys can show a confirmation ("thank you") screen after the last question, but there is no equivalent screen before the first one.

This is the posthog-flutter part of the cross-SDK intro screen rollout tracked in PostHog/posthog#74064. The web/RN renderer shipped in PostHog/posthog-js#4436, and the native SDK parts are in PostHog/posthog-ios#754 and PostHog/posthog-android#677.

What it does:

• New appearance fields parsed from the native payload and exposed on PostHogDisplaySurveyAppearance / resolved in SurveyAppearance: displayIntroScreen, introScreenHeader, introScreenDescription, introScreenDescriptionContentType, introScreenButtonText (button defaults to "Get started").
• New IntroMessage widget, the leading mirror of ConfirmationMessage, shown by SurveyBottomSheet before the first question when displayIntroScreen is on. The description follows the existing text-only rendering rule.
• Advancing past the intro flips local widget state only: no onResponse call, no method-channel surveyAction, so no survey event. The X button keeps closing the survey with the normal dismissed event, and the confirmation branch still wins for completed surveys.
• Both native bridges (PostHogDisplaySurvey+Dict.swift, PostHogDisplaySurveyExt.kt) forward the five new keys, since the appearance maps are hand-enumerated.

⚠️ Blocked on native SDK releases. The bridge code reads the new fields from the native display models, so this PR needs posthog-ios >= 3.70.0 (PostHog/posthog-ios#754) and a posthog-android release containing PostHog/posthog-android#677. The dependency floors are bumped to PostHog >= 3.70.0 and com.posthog:posthog-android:[3.59.0,4.0.0) — please confirm the exact Android artifact version once its release ships (the changesets cascade may produce a different number). CI stays red until both are published; the Dart-only changes are green-path.

💚 How did you test it?

Unit and widget tests added:

• surveys_test.dart: fromDict parses the five intro keys (including the int-coded content type) and defaults to disabled when absent.
• survey_bottom_sheet_test.dart (new): presents the sheet via showModalBottomSheet like SurveyService does, and asserts the intro shows before question 1, advancing fires no onResponse/onClosed callback, the off-flag skips the intro, and closing from the intro still notifies onClosed.

Written on Linux without a Flutter toolchain, so the suite was not executed locally — please rely on CI (flutter test, dart analyze, format checks).

📝 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 (.changeset/survey-intro-screen.md, minor)

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

• Authored with PostHog Code (Claude) as part of the cross-SDK rollout requested in review of feat(surveys): add optional intro screen shown before the first question posthog-js#4436; design per Feature request: Survey intro/start screen (shown before the first question) posthog#74064.
• Key decision: the intro is a late bool state field on _SurveyBottomSheetState initialized from the appearance; its "Get started" handler is a pure setState, deliberately not routed through onResponse/_handleClose so no method-channel call (and therefore no event) can occur on the advance path.

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Created with PostHog Code

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat(surveys): add optional intro screen..." | Re-trigger Greptile}

[github-actions]

posthog-flutter Compliance Report

Date: 2026-08-06 22:50:14 UTC
Duration: 96789ms

✅ 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	✅	142ms
Format Validation.Event Has Uuid	✅	116ms
Format Validation.Event Has Lib Properties	✅	116ms
Format Validation.Distinct Id Is String	✅	114ms
Format Validation.Token Is Present	✅	114ms
Format Validation.Custom Properties Preserved	✅	116ms
Format Validation.Event Has Timestamp	✅	117ms
Retry Behavior.Retries On 503	✅	5330ms
Retry Behavior.Does Not Retry On 400	✅	2118ms
Retry Behavior.Does Not Retry On 401	✅	2117ms
Retry Behavior.Respects Retry After Header	✅	8126ms
Retry Behavior.Implements Backoff	✅	15442ms
Retry Behavior.Retries On 500	✅	5224ms
Retry Behavior.Retries On 502	✅	5221ms
Retry Behavior.Retries On 504	✅	5224ms
Retry Behavior.Max Retries Respected	✅	15441ms
Deduplication.Generates Unique Uuids	✅	124ms
Deduplication.Preserves Uuid On Retry	✅	5222ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10324ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5229ms
Deduplication.No Duplicate Events In Batch	✅	124ms
Deduplication.Different Events Have Different Uuids	✅	115ms
Compression.Sends Gzip When Enabled	✅	115ms
Batch Format.Uses Proper Batch Structure	✅	112ms
Batch Format.Flush With No Events Sends Nothing	✅	108ms
Batch Format.Multiple Events Batched Together	✅	123ms
Error Handling.Does Not Retry On 403	✅	2115ms
Error Handling.Does Not Retry On 413	✅	2117ms
Error Handling.Retries On 408	✅	5219ms

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	✅	10ms
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	✅	9ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	9ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	9ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	11ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	112ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	15ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	9ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	115ms