fix(flags): surface that preloadFeatureFlags is ignored on Flutter web

[posthog]

💡 Motivation and Context

Customers told to set preloadFeatureFlags = false to cut feature-flag request volume see no change on Flutter web. On web, posthog-js is initialized by the host app, so the option (like bootstrap) never reaches the JS SDK — it is a silent no-op. bootstrap at least documents this; preloadFeatureFlags said nothing, so the recommended workaround reads as "the flag didn't work" on the web half of a customer's traffic.

This makes the gap visible rather than silent:

• Web setup() now emits a debug-build warning when preloadFeatureFlags is false (pointing at advanced_disable_feature_flags_on_first_load in posthog.init) or when bootstrap is set.
• preloadFeatureFlags docs carry the same Flutter web: not applied note bootstrap already has.
• identify() and group() docs now say they reload feature flags, so turning preloading off alone does not remove those requests.

Scope note: this does not change request volume on iOS/Android. The bigger win the ticket describes — a cached-flags freshness window so a cold open serves persisted flags instead of refetching — lives in posthog-ios/posthog-android with a passthrough option here, and is not attempted in this PR.

💚 How did you test it?

Added posthog_flutter/test/posthog_flutter_web_setup_test.dart (browser-only) covering: warning emitted when preloadFeatureFlags is false, no warning on the default, and the bootstrap warning.

I was not able to run the test suite or the analyzer — no Flutter toolchain is available in the environment this was authored in, so the tests are unverified and CI is the first real run. Please treat the test as needing a green CI before merge. The rest of the change is doc comments plus two guarded printIfDebug calls.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• Ran pnpm changeset equivalent (changeset file added by hand)

🤖 Agent context

Autonomy: Fully autonomous

Authored by Claude (PostHog Code) from a Signals inbox report about feature-flag request volume. Checked for competing work first — the only open PRs are session replay and surveys work, and no open issue matched.

Decision made along the way: the report's headline fix (a cached-flags freshness window) cannot be implemented in this repo, so this PR deliberately covers only the Flutter-web silent no-op, and adds a runtime warning rather than docs alone so the misconfiguration is discoverable without reading dartdoc.

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Created with PostHog Desktop from this inbox report.

[github-actions]

posthog-flutter Compliance Report

Date: 2026-07-30 17:16:17 UTC
Duration: 96782ms

✅ 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	✅	137ms
Format Validation.Event Has Uuid	✅	118ms
Format Validation.Event Has Lib Properties	✅	114ms
Format Validation.Distinct Id Is String	✅	114ms
Format Validation.Token Is Present	✅	115ms
Format Validation.Custom Properties Preserved	✅	116ms
Format Validation.Event Has Timestamp	✅	115ms
Retry Behavior.Retries On 503	✅	5328ms
Retry Behavior.Does Not Retry On 400	✅	2117ms
Retry Behavior.Does Not Retry On 401	✅	2116ms
Retry Behavior.Respects Retry After Header	✅	8120ms
Retry Behavior.Implements Backoff	✅	15442ms
Retry Behavior.Retries On 500	✅	5224ms
Retry Behavior.Retries On 502	✅	5224ms
Retry Behavior.Retries On 504	✅	5223ms
Retry Behavior.Max Retries Respected	✅	15440ms
Deduplication.Generates Unique Uuids	✅	124ms
Deduplication.Preserves Uuid On Retry	✅	5222ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10334ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5229ms
Deduplication.No Duplicate Events In Batch	✅	126ms
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	✅	123ms
Error Handling.Does Not Retry On 403	✅	2116ms
Error Handling.Does Not Retry On 413	✅	2118ms
Error Handling.Retries On 408	✅	5225ms

Feature_Flags Tests

✅ 16/16 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	13ms
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	✅	10ms
Request Payload.Groups Round Trip	✅	9ms
Request Payload.Groups Default To Empty Object	✅	10ms
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	✅	10ms
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	✅	16ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	9ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	115ms