This is a barebones demo of presence working fine in supabase-js 2.53 when imported from https://cdn.jsdelivr.net/npm/@supabase/supabase-js@2.53/+esm'
Open test2-253.html in a new tab and check the console for both this tab and the new one.
There will be console log acknowledging each other's presence.
Open test1-256.html and test2-256.html in a separate tab.
There will be no console log acknowledging each other's presence, only the basic initialisation logs.
The broken 2.56 pages are 100% identical to these other than the version of supabase-js imported and this accompanying text.