Without a browser key, the app asks its server for a short-lived Google
access token (GET /api/google/tiles-token, answering accessToken and
expiresAt) and loads Photorealistic 3D Tiles directly with it as a
header. A tile refused for an expired token gets a renewed one and is
retried once. A server that does not offer tokens changes nothing: the
app continues through ion or the keyless globe as before.
