fix: static files served with different vary headers (#26532)

posthog/settings/web.py

@@ -251,6 +251,19 @@
 ]
 STATICFILES_STORAGE = "whitenoise.storage.ManifestStaticFilesStorage"
 
+
+def add_vary_headers(headers, _path, _url):
+    """
+    we might serve static content on multiple domains, so we need to vary on Origin
+    as well as accept-encoding to avoid caching issues
+    see: https://github.com/evansd/whitenoise/blob/b3d250fd17da0e280d58b6dc4935c4573ebe8b55/docs/django.rst?plain=1#L392-L422
+    which says: The function should not return anything; changes should be made by modifying the headers dictionary directly.
+    """
+    headers["Vary"] = "Origin, Accept-Encoding"
+
+
+WHITENOISE_STATIC_HEADERS = add_vary_headers
+
 AUTH_USER_MODEL = "posthog.User"
 
 LOGIN_URL = "/login"