fix: enable strictProxyHas for namespace conflict detection in window proxy

[chadgauth]

Use cases and why

This PR fixes a critical compatibility issue with FullStory analytics when loaded via Google Tag Manager (GTM) through Partytown. This is important for users who:

1. Use FullStory analytics with GTM integration - FullStory fails to initialize when GTM injects the script tag with type="text/partytown", blocking session recording and user analytics entirely

2. Need accurate namespace conflict detection in web workers - Any third-party script that checks for property existence using the in operator (e.g., if (!("libraryName" in window))) will benefit from having accurate checks instead of always returning true

3. Require opt-in fixes without breaking existing implementations - Developers can enable strictProxyHas: true only for scripts that need it, while maintaining full backwards compatibility with existing Partytown configurations

Problem Statement

When FullStory is loaded via GTM's Custom HTML tag through Partytown, it fails to initialize with a "namespace conflict" error. This occurs because:

1. FullStory's initialization script checks: if (!("FS" in window))
2. Partytown's window proxy had has: () => true - always returning true regardless of actual property existence
3. This false positive causes FullStory to think "FS" already exists, triggering the namespace conflict protection

Solution

Added a new configuration option strictProxyHas that allows accurate namespace conflict detection:

• When true: The in operator uses Reflect.has() for accurate property checks
• When false (default): Maintains existing behavior for backwards compatibility
• Opt-in approach prevents breaking existing implementations

Changes Made

Code

• src/lib/types.ts - Added strictProxyHas?: boolean to PartytownConfig with JSDoc
• src/lib/web-worker/worker-window.ts - Updated window proxy's has trap to conditionally use Reflect.has()

Tests

• tests/integrations/full-story/full-story.spec.ts - Updated with new "full-story via GTM" test case
• tests/integrations/full-story/gtm-fullstory.html - New integration test using exact production FullStory snippet from GTM

Documentation

• docs/src/routes/configuration/index.mdx - Added strictProxyHas to configuration table
• docs/src/routes/common-services/index.mdx - Added FullStory with GTM configuration guide and dedicated section

Infrastructure

• .gitignore - Added test-results/ to prevent committing Playwright test artifacts
• .changeset/strict-proxy-has.md - Changeset for version bump

Testing

✅ FullStory GTM Integration Test: New test passes with strictProxyHas: true
✅ Full Test Suite: 55/57 tests passing (2 pre-existing failures in error handling and mutation-observer, unrelated to this change)
✅ No Regressions: All previously passing tests continue to pass
✅ Backwards Compatible: Default behavior (strictProxyHas: false) unchanged

How Users Can Use This

<script>
  partytown = {
    forward: ['FS.identify', 'FS.event'],
    strictProxyHas: true  // Enable accurate 'in' operator for FullStory
  };
</script>
<script src="/~partytown/partytown.js"></script>

Checklist:

• My code follows the developer guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• Added new tests to cover the fix / functionality

[changeset-bot]

🦋 Changeset detected

Latest commit: c251ee3

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@qwik.dev/partytown	Minor

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[chadgauth]

The CI failure is a main branch issue (pre-existing formatting problems) and not related to my changes.

[thejackshelton]

LGTM as a nice user facing bug fix. Note: The CI lint failure is pre-existing on main, not related to this PR. Will merge and release soon.

Also apologies for the wait @chadgauth 😅 . Will be more active going forward.