Set Posthog Cookie on site domain, not mit.edu

[ChristopherChudzicki]

What are the relevant tickets?

For

• https://github.com/mitodl/hq/issues/11158

Description (What does it do?)

Set the posthog cookie on site domain (open.odl.local, or learn.mit.edu, or rc.learn.mit.edu) not MIT domain.

How can this be tested?

1. With Posthog integrated locally, you should see a ph_PROJECT_TOKEN_posthog cookie set on your site domain exactly:
   • if site domain is open.odl.local, it should be set there, not on odl.local
   • if site domain is learn.mit.dev, it should be set there, not on mit.dev

Additional Context

Setting the cookie on the site domain loses our ability to track anonymous (unauthenticated) users across sites within the same project, unless we do special setup like pass distinct_ids via query params in URLs, but that is deemed acceptable.

Companion PR, same change for the Keycloak login pages: mitodl/ol-keycloakify#176

mitxonline already does this — it has passed cross_subdomain_cookie: false since 2023: https://github.com/mitodl/mitxonline/blob/80c2a512f1e3d2a93c06bb4270ba87976cf369ef/frontend/public/src/lib/util.js#L44

[github-actions]

OpenAPI Changes

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Unexpected changes? Ensure your branch is up-to-date with main (consider rebasing).

[Copilot]

Pull request overview

Updates the PostHog client initialization in the Next.js frontend so analytics cookies are scoped to the current site domain (e.g., learn.mit.edu, rc.learn.mit.edu, open.odl.local) instead of being shared across subdomains via the parent domain.

Changes:

• Disable PostHog cross-subdomain cookies by setting cross_subdomain_cookie: false during posthog.init.

[daniellefrappier18]

LGTM

[zawan-ila]

We may also need this change in https://github.com/mitodl/ocw-hugo-themes/blob/f88bfc52184d19a7f366b65f225750602ccef6a5/base-theme/assets/js/posthog.ts#L18-L28, perhaps just scoped to the v3 theme? The v3 theme pages are served at urls like https://learn.mit.edu/courses/o/21h-151-dynastic-china-fall-2024/ Currently, they are "gated" behind the ocw-product-pages flag.