feat: Treat self referrals as direct

[robbie-c]

Changes

We handle the $referrer property in a potentially confusing way when an internal link is clicked. The latest example is https://posthog.slack.com/archives/C03C60FT1J7/p1726670676997629.

This happens because we use document.referrer directly, and this can be self-referential in the following example

1. User is on https://referrer.com
2. User clicks a link to https://tracked.com
3. Referrer is referrer.com
4. User is inactive long enough for the session to expire
5. User clicks an internal link to https://tracked.com
6. Referrer is tracked.com

This PR changes this, so that the referrer in 6) would be $direct

To be clear about this: it is a breaking change, however I believe it's more in line with what the expected behaviour would be. One might argue that the original behaviour is a bug

Checklist

• Tests for new code (see advice on the tests we use)
• Accounted for the impact of any changes across different browsers
• Accounted for backwards compatibility of any changes (no breaking changes in posthog-js!)

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Sep 20, 2024 10:00am

[github-actions]

Size Change: +303 B (+0.02%)

Total Size: 1.21 MB

Filename	Size	Change
dist/array.full.js	347 kB	+75 B (+0.02%)
dist/array.js	163 kB	+76 B (+0.05%)
dist/main.js	164 kB	+76 B (+0.05%)
dist/module.js	163 kB	+76 B (+0.05%)

ℹ️ View Unchanged

Filename	Size
dist/exception-autocapture.js	10.4 kB
dist/recorder-v2.js	111 kB
dist/recorder.js	111 kB
dist/surveys-preview.js	59.8 kB
dist/surveys.js	66 kB
dist/tracing-headers.js	8.26 kB
dist/web-vitals.js	10.3 kB

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[pauldambra]

Asked a question about this in the private slack posthog.slack.com/archives/C03C60FT1J7/p1726670676997629

[robbie-c]

Closing PR following discussion on slack

[robbie-c]

See PostHog/posthog.com#9408 (review) instead