Unfortunately on safari the session does not stay logged in once a user closes the tab. This is due to updates to how Safari stores cookies. It doesn’t allow storing cookies from different domains for more than one session, and google’s client side login requires a different domain cookie.
The stop-gap fix is for users to go to:
The same applies for mobile:
A long term fix will be to implement server-side login, or if gapi.client gets fixed it will be irrelevant.
I’ll look into this again towards the end of the year if it hasn’t been looked at by Google yet.
Details:
https://github.com/google/google-api-javascript-client/issues/342
Example: