fix(push): capture $push_notification_opened on iOS cold start

[turnipdabeets]

💡 Motivation and Context

Fixes #555 — $push_notification_opened is never captured on iOS. Reproduced on a simulator; two independent causes:

1. Nobody is listening. A stock Flutter app sets no UNUserNotificationCenter delegate, so iOS reports the tap to nobody and the native SDK's swizzling has nothing to attach to. (Note flutter_local_notifications does not set it either — it registers as an application delegate and relies on FlutterAppDelegate forwarding.)
2. We start too late. On a cold launch from a tap, didReceive arrives ~150 ms in, about 90 ms before Dart reaches Posthog().setup().

⚠️ Blocked on PostHog/posthog-ios#792 — this cannot compile until that ships PostHogSDK.prewarmPushNotificationOpenCapture(). The podspec and Package.swift floors are already raised to >= 3.72.0, so merging or releasing this before posthog-ios 3.72.0 is published fails pod install / SPM resolution for every iOS user — a hard build break, not a degradation. Draft until then.

💚 How did you test it?

On device — posthog-flutter example app, iPhone 17 Pro sim / iOS 26.4, built against a local posthog-ios with the native change:

• 4/4 notification taps captured, one event each (1 warm start, 3 cold starts), confirmed in ingestion. Before the fix, cold starts captured 0 and warm starts captured 0 for want of a delegate.
• Warm start captures exactly once — no double-counting from the new buffered path.
• With the delegate removed, the new native warning fires; with it set, it does not.

flutter analyze clean. The native unit tests live in the posthog-ios PR.

Testing

Verified on an iPhone 17 Pro simulator (iOS 26.4) against a local posthog-ios build, with events confirmed in a real PostHog project:

Scenario	Result
Cold launch from a tap	captured (was 0 before this change)
Tap while the app is running (warm)	captured
Launch with no push payload	not captured
capturePushNotificationOpened: false	not captured, and the integration never installs
Example AppDelegate with the delegate line removed	the new debug warning fires
Delegate present	no warning

Repeated across four rounds — 15 $push_notification_opened events ingested in total, carrying $notification_title and $notification_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

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Built with Claude Code (session), driven by @turnipdabeets. Nearly all the logic lives in posthog-ios; this side is the trigger plus the example wiring.

Worth a reviewer's attention:

• prewarmPushNotificationOpenCapture() is called from register(with:) because that runs inside didFinishLaunchingWithOptions. AUTO_INIT would also win the race (verified), but it is mutually exclusive with pushIdentityProvider, so it isn't a usable answer for everyone.
• The Info.plist key is the only opt-out reachable that early — a Dart-side capturePushNotificationOpened: false isn't known yet. The native SDK releases an unwanted prewarm at setup(), so this only matters for one add-to-app ordering, documented at the call site. That widens the key's scope beyond AUTO_INIT apps, hence the second changeset.
• The example AppDelegate change is the fix for cause (1) and is the copy-pasteable bit for users hitting this.

Not done here, deliberately: a README/docs section (the SDK currently ships push with no iOS setup docs at all). The identical cold-start race exists in @posthog/react-native-plugin and needs a follow-up issue on posthog-js.

[github-actions]

posthog-flutter Compliance Report

Date: 2026-09-02 15:50:53 UTC
Duration: 96644ms

✅ All Tests Passed!

45/45 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	132ms
Format Validation.Event Has Uuid	✅	117ms
Format Validation.Event Has Lib Properties	✅	115ms
Format Validation.Distinct Id Is String	✅	114ms
Format Validation.Token Is Present	✅	114ms
Format Validation.Custom Properties Preserved	✅	115ms
Format Validation.Event Has Timestamp	✅	115ms
Retry Behavior.Retries On 503	✅	5325ms
Retry Behavior.Does Not Retry On 400	✅	2116ms
Retry Behavior.Does Not Retry On 401	✅	2117ms
Retry Behavior.Respects Retry After Header	✅	8124ms
Retry Behavior.Implements Backoff	✅	15444ms
Retry Behavior.Retries On 500	✅	5225ms
Retry Behavior.Retries On 502	✅	5225ms
Retry Behavior.Retries On 504	✅	5224ms
Retry Behavior.Max Retries Respected	✅	15437ms
Deduplication.Generates Unique Uuids	✅	121ms
Deduplication.Preserves Uuid On Retry	✅	5223ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10334ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5228ms
Deduplication.No Duplicate Events In Batch	✅	121ms
Deduplication.Different Events Have Different Uuids	✅	114ms
Compression.Sends Gzip When Enabled	✅	113ms
Batch Format.Uses Proper Batch Structure	✅	113ms
Batch Format.Flush With No Events Sends Nothing	✅	108ms
Batch Format.Multiple Events Batched Together	✅	121ms
Error Handling.Does Not Retry On 403	✅	2114ms
Error Handling.Does Not Retry On 413	✅	2116ms
Error Handling.Retries On 408	✅	5222ms

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	✅	8ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	8ms
Request Payload.Flags Request Omits Authorization Header	✅	8ms
Request Payload.Token In Flags Body Matches Init	✅	7ms
Request Payload.Groups Round Trip	✅	8ms
Request Payload.Groups Default To Empty Object	✅	8ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	8ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	7ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	7ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	8ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	110ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	14ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	8ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	113ms