feat: add posthog distinctid support

frontend/src/oso.tsx

@@ -1,21 +1,45 @@
 /* Copyright 2024 Marimo. All rights reserved. */
 
-import posthog from 'posthog-js'
+import posthog from "posthog-js";
 import { mount } from "@/oso-mount";
 
-
 declare global {
   interface Window {
     __MARIMO_MOUNT_CONFIG__: unknown;
   }
 }
 
 if (import.meta.env.VITE_POSTHOG_PUBLIC_API_KEY) {
+  // Read identity passed from the parent page via URL fragment to stitch events
+  // under the same distinct_id without emitting a spurious $identify event.
+  const fragment = new URLSearchParams(window.location.hash.slice(1));
+  const distinctID = fragment.get("posthogDistinctId") ?? undefined;
+  const sessionID = fragment.get("posthogSessionId") ?? undefined;
+
+  // Strip the identity params from the URL once consumed, so they don't linger
+  // in a shareable/openable link and stitch events under the wrong identity.
+  // Only the PostHog keys are removed; any other fragment state is preserved.
+  if (distinctID || sessionID) {
+    fragment.delete("posthogDistinctId");
+    fragment.delete("posthogSessionId");
+    const rest = fragment.toString();
+    history.replaceState(
+      null,
+      "",
+      window.location.pathname +
+        window.location.search +
+        (rest ? `#${rest}` : ""),
+    );
+  }
+
   posthog.init(import.meta.env.VITE_POSTHOG_PUBLIC_API_KEY, {
+    bootstrap: distinctID
+      ? { distinctID, sessionID, isIdentifiedID: true }
+      : undefined,
     session_recording: {
       // WARNING: Only enable this if you understand the security implications
       recordCrossOriginIframes: true,
-    }
+    },
   });
 } else {
   console.warn("POSTHOG_PUBLIC_API_KEY not set, skipping posthog init");